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LEAF TOUGHNESS AS A MEASURE OF DECOMPOSITION RATES OF SELECTED TREE SPECIES IN THE RIVER NJORO, KENYA

MWANAKE HOPE WAKIO

B.Sc. APPLIED AQUATIC SCIENCE

S14/20020/06

hmwanake@yahoo.coms

A research project report submitted to the Faculty of Science, Biological Sciences Department in partial fulfillment of the requirements for the Degree of Bachelor of Science in Applied Aquatic Science.

EGERTON UNIVERSITY

April, 2010


ABSTRACT


The litter breakdown of riparian leaves in lotic systems has been intensely studied in the past 30 years in temperate regions and it is now clear that allochthonous organic matter, originating in the riparian zone, is a main energy source to many low order forested streams (Hutchens and Wallace, 2002). Once in the stream channel, the leaf litter can either be entrained by the water current or retained in the humid zone (Mathooko et al., 2000a).

Previous researches on River Njoro have concentrated more on the distribution of detritus as well as macro invertebrates, litter processing and microbiology while ignoring to point out the relationship between leaf toughness and decomposition as well as with macro invertebrates abundance and diversity. It is therefore the principle aim of this study to investigate the relationship between leaf toughness and litter decomposition as well as macro invertebrates abundance in River Njoro.

The specific objectives were to compare leaf toughness as a measure of decomposition rates of Dombeya goetzenii, Syzygium cordatum, Rhus natalensis and Vanguera madagascariensis, to determine the correlation between changes in leaf toughness in weight and dry weight of the leaf species and to determine the correlation between changes in leaf toughness in weight and numbers of the macro invertebrates in the River Njoro.

The study site was River Njoro and the sampling site was treetops. The results indicated that among the four leaf species Syzygium cordatum and Rhus natalensis decomposed slowly and their measurement of leaf toughness were highest as compared to Vanguera madagascariensis and Dombeya goetzenii whose leaf litter disappeared by day 16 and their measurements of leaf toughness proved that the two were the softest leaf species. There were strong positive correlations between dry weight and leaf toughness in all the four leaf species.

A related study needs to be carried out during a rainy season in order to compare the differences in the leaf toughness, Furthermore many other topics related to litter breakdown and leaf toughness still need to be investigated in River Njoro, including the life history of true shredders, the capability of other invertebrates to feed on decomposing leaves, the presence and abundance of mining shredders the variability of leaf litter input to streams.

DECLARATION


This report is my original work and has not, wholly or in part, been presented for an award of a degree in any other university.

Mwanake Hope Wakio

Signed:

Date:



SUPERVISOR’S RECOMMENDATION

This report is the candidate’s original work and has been prepared with my guidance and assistance for the partial fulfillment of the course Aquatic Research Project (AQUA 470).

Dr. Charles Mwithali M’Erimba

Department of Biological Sciences

Egerton University

Signed:

Date:

DEDICATION


To my parents George and Rhoda Mwanake, my brothers Ricky, Nickson and Billy Mwanake. My world revolves around you; I am all I am because of your love. I love you

Leo thank you for supporting me through this endless journey.



ACKNOWLEDGEMENTS


I would like to thank my supervisor Dr. Charles Mwithali M’Erimba who gave me invaluable guidance and advice during the entire research period. He also tirelessly worked with me. I am grateful for his constructive criticisms and contribution that lead to the successful completion of the project.

Special thanks to Racheal Njoroje and Mungai of the Department of Biological Sciences for constantly encouraging me and staying with me up to late hours and during weekends during the entire project period.

I am greatly indebted to Mr. Henry Karanja who sacrificed his time to offer me field support in the river without him my project would have been incomplete. I am also greatly indebted to Mr. John Gichimu Mbaka who tirelessly worked with me during the analysis of my data.

I would like to extend my gratitude to the entire staff of Biological Sciences Department Dr. Magana, Mr Eddison, Mr. Ongondo, Mr. Obongo, Mr. Obwanga, Mr. Otachi, Madams Jane, Risper and Nancy all who made my project a success.

Lastly I would like to thank my parents Mr. and Mrs. George and Rhoda Mwanake, my brothers Ricky, Nickson and Billy Mwanake for providing me with support, vision and encouragement throughout the entire project period.

TABLE OF CONTENTS


ABSTRACT ii

DECLARATION iii

DEDICATION iv

ACKNOWLEDGEMENTS v

TABLE OF CONTENTS vi

LIST OF TABLES vii

LIST OF FIGURES viii

CHAPTER ONE ix

1.0 INTRODUCTION ix

1.1 Background Information ix

1.2 Statement of the Problem x

1.3 Study Objectives xi

1.3.1 Broad Objectives xi

1.3.2 Specific Objectives xi

1.4 Research Hypothesis xi

1.5 Justification of the study xii

CHAPTER TWO xiii

2.0 LITERATURE REVIEW xiii

2.1 Macro invertebrates and their role in lotic ecosystems xiii

2.1.1 Role of Macro invertebrates in Bio-monitoring xiii

2.1.2 Role of macro invertebrates in Coarse Particulate Organic Matter (CPOM) processing in streams xiii

2.1.3 Role of macro invertebrates in aquatic food chains xiv

2.2 Litter decomposition in a Cerrado savannah stream xiv

2.3 Breakdown of leaf litter in a tropical stream xv

2.4 Allochthonous detritus as a main source of energy for woodland stream ecosystems xv

2.5 Leaf litter processing rates in a Kenyan highland stream the River Njoro xvi

CHAPTER THREE xvii

3.0 Materials and Methods xvii

3.1 Study area xvii

3.2 Study site xviii

3.3 Sampling and analysis protocol xix

3.3.1 Environmental variables xix

3.3.2 Leaf litter sampling and laboratory sample processing xix

3.4 Assumptions of the study xx

3.5 Limitation of the study xx

CHAPTER FOUR xxi

4.0 Results xxi

CHAPTER FIVE xxix

5.0 Discussions of the results xxix

5.1 River Njoro Physico-chemical parameters xxix

5.2 Comparison of leaf toughness among the four leaf species xxix

5.3 Correlation between numbers of organisms and leaf toughness among the four species xxx

CHAPTER SIX xxxii

6.0 Conclusions and Recommendations xxxii

6.1 Conclusions xxxii

6.2 Recommendations xxxiii

References xxxiii

APPENDICES xxxv


LIST OF TABLES


Table 1 Physico-chemical parameters measured on each sampling date xxi

Table 2 Analysis of one way ANOVA xxiv




LIST OF FIGURES


Figure i: Location of the River Njoro and its catchment in Kenya xviii

Figure ii: Rate of leaf toughness for Syzygium cordatum as determined by Penetrometer at diameter 0.79mm and 1.55mm xxii

Figure iii: Rate of leaf toughness for Dombeya goetzenii as determined by Penetrometer at diameter 0.79mm and 1.55mm xxii

Figure iv: Rate of leaf toughness for Vanguera madagascariensis as determined by Penetrometer at diameter 0.79mm and 1.55mm xxiii

Figure v: Rate of leaf toughness for Rhus natalensis as determined by Penetrometer at diameter 0.79mm and 1.55mm xxiii

Figure vi: Comparison of changes in leaf toughness with time of exposure for Dombeya, Vanguera, Rhus and Syzigium as determined by Penetrometer at diameter 0.79mm xxv

Figure vii: Shows correlation between numbers of organisms and leaf toughness of Syzigium as determined by diameter 0.79mm xxv

Figure viii: Shows correlation between numbers of organisms and leaf toughness of Rhus as determined by diameter 0.79mm xxvi

Figure ix: Shows correlation between numbers of organisms and leaf toughness of Dombeya as determined by diameter 0.79mm xxvi

Figure x: Shows correlation between numbers of organisms and leaf toughness of Vanguera as determined by diameter 0.79mm xxvii

Figure xi: Shows correlation between dry weight and leaf toughness of Vanguera as determined by diameter 0.79mm xxvii

Figure xii: Shows correlation between dry weight and leaf toughness of Dombeya as determined by diameter 0.79mm xxviii

Figure xiii: Shows correlation between dry weight and leaf toughness of Rhus as determined by diameter 0.79mm xxviii

Figure xiv: Shows correlation between dry weight and leaf toughness of Syzigium as determined by diameter 0.79mm xxix




CHAPTER ONE

1.0 INTRODUCTION

1.1 Background Information


The litter breakdown of riparian leaves in lotic systems has been intensely studied in the past 30 years in temperate regions and it is now clear that allochthonous organic matter, originating in the riparian zone, is a main energy source to many low order forested streams (Hutchens and Wallace, 2002). Once in the stream channel, the leaf litter can either be entrained by the water current or retained in the humid zone (Mathooko et al., 2000a).

Organic matter in the river occurs mainly from the aerial and lateral inputs (Magana, 2000). The balance of the export and import of the leaf litter determines the amount of leaves deposited in a stream channel in time and space. Furthermore, the duration of stay of the leaf litter in a certain area of the stream channel determines the density and composition of the stream biocoenosis that invades the litter as well as the intensity of the biofilm that forms on it (Mathooko, 1995).

The leaves thus conditioned are rich in protein and hence assumed to be most palatable to the stream fauna. The processing of the leaf litter takes place partly on the sediment surface and partly on the bed sediments. The breakdown rates of leaves in streams are affected by external factors such as temperature, physical abrasion, pH, nutrient availability and presence of consumers. (Goncalves et al., 2007) Leaf intrinsic factors can also be used to predict breakdown rates. These include leaf nutrient content, hardness and the presence of defensive compounds. The intrinsic properties of leaves are strongly related to local environmental conditions for example some have hard leaves, which may present particular difficulties to herbivores.

Previous researches on River Njoro have concentrated more on the distribution of detritus as well as macro invertebrates, litter processing and microbiology while ignoring to point out the relationship between leaf toughness and decomposition as well as with macro invertebrates abundance and diversity. It is therefore the principle aim of this study to investigate the relationship between leaf toughness and litter decomposition as well as macro invertebrates abundance in River Njoro.


1.2 Statement of the Problem


The rates of breakdown leaves in streams are affected by the leaf intrinsic factors including toughness and the presence of defensive compounds. Leaf toughness slows the decomposition rates of leaves, as a result creating a major problem to stream bioenergetics. This is because leaf litter has over the last few decades been emphasized as the main energy basis for low order streams like River Njoro since leaves in the wet zone represent current source of detritus at the site of retention and also for the different downstream reaches while those on the humid zone represent potential energy subsidy to the stream ecosystem. River Njoro therefore needs more studies to investigate leaf toughness.

1.3 Study Objectives

1.3.1 Broad Objectives


The aim of the study will be primarily to determine leaf toughness as a measure of decomposition rates of Dombeya goetzenii, Syzygium cordatum, Rhus natalensis and Vanguera madagascariensis in a Kenyan highland stream, the River Njoro.

1.3.2 Specific Objectives


  1. To compare leaf toughness as a measure of decomposition rates of Dombeya goetzenii, Syzygium cordatum, Rhus natalensis and Vanguera madagascariensis in the River Njoro.

  2. To determine the correlation between changes in leaf toughness in weight and dry weight of the leaf species in the River Njoro.

  3. To determine the correlation between changes in leaf toughness in weight and numbers of the macro invertebrates in the River Njoro.

1.4 Research Hypothesis


  1. H0: There is no difference in leaf toughness and decomposition rates of Dombeya goetzenii, Syzygium cordatum, Rhus natalensis and Vanguera madagascariensis in the River Njoro.

  2. H0: There is no correlation between changes in leaf toughness in weight and dry weight of the leaf species in the River Njoro.

  3. H0: There is no correlation between changes in leaf toughness in weight and numbers of the macro invertebrates in the River Njoro.

1.5 Justification of the study


Despite the crucial role played by allochthnous organic matter in the energy dynamics of low order streams, there is very little information on the decomposition of leaf litter and other organic matter in relation to the leaf toughness in the tropical streams like River Njoro. Most of the existing information about litter breakdown and toughness in aquatic systems comes from experiments carried out in temperate zones. Furthermore comparisons of results gathered in different geographic areas and biomes can be questionable, as differences in decay rates can be attributed to differences in climate but also differences in the quality of litter.

It is with this rationale and limited information on leaf toughness that prompted the undertaking of this study. This study is also of relevance to stream bioenergetics in that the high rate of decomposition releases nutrients at the leaves decomposition site and also the downstream reaches. This is very important to the local neighboring community since it improves the stream health. Furthermore, decomposition also reduces the accumulation of leaf litter on the streambed making the water clear and clean for use in domestic and recreational purposes.


CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 Macro invertebrates and their role in lotic ecosystems

2.1.1 Role of Macro invertebrates in Bio-monitoring


Bio-monitoring is the systematic use of living organisms or their response to determine the quality of environment (Kar and Chu, 1998). When water quality, habitat conditions and land use are evaluated along with macro invertebrates, the result is a comprehensive picture of environmental quality (Cao et al., 1996). This information helps resource managers to determine which streams and rivers have good habitat quality and should be protected. The benthos can also help identify those rivers and streams showing signs of stress. Resource managers can then apply management action that will improve environmental quality, such as storm water control in urban areas and best practices on farm lands to control nutrient runoff (Cao et al., Op. Cit.)

Unlike fish, macro invertebrates cannot move around so much they are able to escape the effects of sediment and other pollutants that diminish water quality. The long life cycles of macro invertebrates are between six months and two years for worms, up to five years for dragon flies. This allows studies conducted by aquatic ecologists to determine any decline in environmental quality. Macro invertebrates represent an extremely diverse group of aquatic animals and the large numbers of species posses a wide range of responses to stressors such as organic pollutants, sediments and toxicants (Cao et al., Op. Cit.).


2.1.2 Role of macro invertebrates in Coarse Particulate Organic Matter (CPOM) processing in streams


The consumption of autumn shed leaves in woodland streams by various invertebrates is the most extensively investigated pathway involving CPOM. The breakdown of CPOM culminates in a limited number of possible fates: mineralization, storage and export. However, much of the CPOM becomes fine particulates, which are difficult to follow, and so the dynamics of CPOM once it becomes fine particulate organic matter are not well understood (Allan, 1995). Invertebrates that feed on decaying leaves include crustaceans (especially amphipods, isopods, crayfish and fresh water shrimp), snails and several groups of insect larvae. The latter include cranefly larvae (Tipulidae), and several families of trichopterans (Limnephilidae, Lepidostomatidae, Sericostomatidae, Oeconesidae) and plecopterans (Peltoperlidae, Pteronarcidae, Nemouridae) (Allan Op. Cit.) The leaf shredding activities of insect larvae and gammarid amphipods are particularly well studied (Allan Op. Cit.) Tipula and many Limnephilid caddis larvae eat all parts of the leaf, both mesophyll and venation, whereas Peltoperlid stonefly nymphs avoid venation and concentrate mainly on mesophyll, cuticle and epidermal cells. The snails and Gammarus scrape the softer tissue while the bigger crustaceans are able to tear and engulf larger leaf fragments (Allan Op. Cit.).

2.1.3 Role of macro invertebrates in aquatic food chains


Macro invertebrates play an important role in the aquatic food chain. Many invertebrates feed on algae and bacteria, which are on the lower end of the food chain. Some shred and eat leaves and other organic matter that enters the water. Because of their abundance and position as “middle men” in the aquatic food chain, macro invertebrates play a critical role in the natural flow of energy and nutrients. As invertebrates die, they decay leaving behind nutrients that are reused by aquatic plants and other animals in the food chain (Kar and Chu, 1998).

2.2 Litter decomposition in a Cerrado savannah stream


A study by Goncalves et al (2007) investigated the breakdown rates of Protium brasiliense in the tropical Cerrado stream and found out that the rates were slow, according to the classification of Petersen and Cummins (1974) lower than in the temperate stream and lower than those reported for other leaf species in temperate (Ostrofsky, 1997), Mediterranean (Maamri et al 1997) and tropical African and American streams (Mathuriau and Chauvet, 2002). These slow rates were mainly related to two independent factors: intrinsic leaf characteristics and environmental factors. Physical and chemical properties of P. brasiliense i.e. the leaf nutrient content especially nitrogen content was predicted to have an effect on the breakdown rates. Senescent leaves of P. brasiliense had 2.31% N, which was within the range of values reported for 47 species by Flindt and Lillebø, (2005) whereas the 0.60% of P was in the lower range of 28 species mentioned by the same authors. The nutrient content of P. brasiliense leaves was therefore within the range of the published data.

Lignin was also used to predict the breakdown rates. Leaves of P. brasiliense contained 26% of lignin, which was in the upper range of values reported by Gessner (2005) Lignin is a structural constituent conferring toughness to leaves, protecting them from herbivory, microbial infections and providing waterproofing properties to plants cell walls; it is particularly difficult to biodegrade and limits the degradation of other plant cell compounds. The high lignin content in P. brasiliense and other Cerrado plants was a consequence of the water and nutrient stress of the Cerrado and may explain the low herbivory in the system. Leaf toughness therefore is an explanation for the slow breakdown rates observed in the tropical Cerrado streams reported in this and other related studies (Goncalves et al., 2006).


2.3 Breakdown of leaf litter in a tropical stream


A study by Mathuriau and Chauvet (2002) investigated the breakdown of two leaf species, Croton gossypifolius and Clidemia sp in a 4th order tropical stream (Andean mountains, southwestern Columbia) using leaf bags over a 6 week period. They determined the initial leaf chemical composition and followed the change in content of organic matter, C, N and ergosterol, the sporulation activity of aquatic hyphomycetes, the structure and composition of leaf associated aquatic hyphomycetes and macro invertebrates. Both leaf species Croton gossypifolius and Clidemia sp decomposed rapidly k=0.0651 and 0.0235 respectively. Croton lost 95% of its initial mass within 4 weeks compared to 54% for Clidemia. These rates were related to leaf toughness and to the stable and moderately high water temperature (190C) favoring strong biological activity (Mathuriau and Chauvet, 2002).

2.4 Allochthonous detritus as a main source of energy for woodland stream ecosystems


Once in the stream this detritus is subject to breakdown by a combination of physical and biological processes leading to size reduction, chemical transformation and incorporation into the food web (Petersen and Cummins, 1974). In temperate streams, both micro fungi and shredders are important leaf decomposers because they convert a major part of plant detritus to carbon dioxide, dissolved organic matter, fine particulate organic matter and living biomass. Some of these particulate fractions are further used by other macro invertebrates, so they play an important role in the trophic dynamics of streams (Egglishaw, 1964).

Studies in low order tropical streams generally report rapid breakdown of leaf litter compared to temperate rivers (Dudgeon, 1982). He observed that the higher invertebrate densities on a fast decomposing leaf species were related to decreased leaf toughness and the higher abundance of the leaf associated micro flora and therefore shredders were of secondary importance.


2.5 Leaf litter processing rates in a Kenyan highland stream the River Njoro


Shaded stream channels are heavily depended upon detritus derived from the adjacent terrestrial environment for their energy resources and established theory states that a key component in its processing is the activity of large invertebrate shredders. However shredding invertebrates normally occur at much lower densities in tropical streams relative to those in temperate zone, suggesting a qualitative difference between streams in temperate and tropical regions. The reason for the paucity of shredders is unclear, as tropical stream channels often contain plenty of potential food resources for this guild. For example, Dobson et al (2002) demonstrated high standing stock of detritus in several Kenyan highland streams and proposed a series of possible explanations for the absence or low density of shredders.

One of these related to leaf litter quality; they speculated that riparian trees in Kenya produce leaf litter that is much more refractory and therefore of considerably lower nutritional value than that produced by riparian trees in the North Temperate Zone. This proposal was based upon little more than a cursory examination of leaves from the dominant riparian trees, but it was supported by Mathooko et al (2002c) who determined a slow decay rate for leaves of the common riparian tree Syzygium cordatum. In contrast it was apparently contradicted by a second study on this river, carried out on leaves of another common riparian tree, Dombeya goetzenii, which demonstrated a high decomposition rate among leaves permanently submerged within the river channel (Mathooko et al 2000b). This later study had, however, been carried out to compare decomposition rates in aquatic and terrestrial situations and as the authors stated their initial preparation of leaves may have altered natural decomposition processes.


CHAPTER THREE

3.0 Materials and Methods

3.1 Study area


The study was conducted in the river Njoro (Figure 1) The River Njoro is in Nakuru District, Rift valley province. It is a second order river with a low gradient. It originates from the Eastern Mau hills (2700 m.a.s.l). The river is approximately 60 kilometers (km) in length. It emanates from the Eastern slopes of Mau Escarpment at about 3000 m.a.s.l and terminates in Lake Nakuru at about 1750 meters above sea level and little shuru is its main tributary. Its catchment (Lat. 0015’S, 0025’S; Long. 35050’E, 36005’E) is approximately 200km2. The river passes through dense human settlements including Sigotik, Njokerio, Njoro Township, Egerton University, Nakuru town, high density and low density residential areas. The watershed of about 280km2 has over 300,000 people and includes the urban centers of Njoro Town (30,000) and much of Nakuru Municipality (240,000). On its journey from its source, the river cuts across several land uses and communities with diverse cultural orientations.

The middle course (Egerton University campus area) shows a typical riffle-pool sequence. The soils in the catchment are predominantly of volcanic clay loam, except near the lake where silt clay is found. The area receives annual rainfall of between 760-1270mm and experiences bimodal pattern with long rains in April-June and short rains from July-August. The average temperature is 16.50C and varies with altitude.





Figure i: Location of the River Njoro and its catchment in Kenya

Source: Egerton University Geography laboratory


3.2 Study site


The study site was Tree tops (Figure 1) Its location is S 00° 22.549‘ E 35° 55.162‘, an elevation of 2,307 m.a.s.l and distance from mouth is ~32 km. The land use at Tree tops is farming. It was heavily shaded by trees of Dombeya goetzenii, Syzygium cordatum, Rhus natalensis and Vanguera madagascariensis. All are common evergreen riparian species and account for most of the tree species encountered in the area (Magana, 2000)

3.3 Sampling and analysis protocol

3.3.1 Environmental variables


Water conductivity, salinity and temperature were measured with a conductivity meter; pH with a pH meter; Dissolved oxygen and percentage oxygen saturation were measured using an oxygen meter. Measurements were taken during every sampling period.

3.3.2 Leaf litter sampling and laboratory sample processing


Leaves of Dombeya goetzenii, Syzygium cordatum, Rhus natalensis and Vanguera madagascariensis were collected from the trees and dried at 600C for 48hrs. Each of the dried leaf species was tested for leaf toughness using a penetrometer with the 0.79mm diameter and the 1.55mm diameter. For each diameter each leaf species had six replicates of leaf toughness measured and recorded. The remaining dried leaves from each species were then placed into litterbags (130 × 120 × 20mm), mesh size 5 × 5mm. Each mesh bag contained approximately 4g of one species of leaf and a name label of alluminium foil. Sixty bags in total were made, twelve bags from each species (three replicates from each leaf species). The bags were then arranged into sets of four, each set containing one bag from each species. The sets were then tied onto a string, far apart enough apart to avoid overlap among bags. Five strings were secured across the stream channel in the water by tying to tree roots far apart enough apart to avoid bags from different strings coming into contact with each other. Each string had three sets. Leaves were not pre-leached. The strings with the bags were placed into the stream on 13th November 2009 and five strings randomly chosen were removed after 1, 4, 8, 16 and 36 days without disturbing the others during retrieval. The last string was retrieved on 14th December 2009.

Upon removal, each mesh bag was handled separately leaves were carefully cleaned of extraneous material and all the macro-invertebrates from the leaves, mesh bag and water were removed, placed in a petri-dish with clean water, counted using a dissecting microscope then placed into 5% formaldehyde. The leaves from each of the twelve mesh bags were dried separately at 600C for 48hrs, reweighed and the weight recorded. Each of the retrieved dried leaf species was tested for leaf toughness using a penetrometer with the 0.79mm diameter and the 1.55mm diameter. For each diameter each leaf species had six replicates of leaf toughness measured and recorded. These processes were repeated during each of the sampling days.


3.4 Assumptions of the study


In this study it was assumed that physical conditions in the study site offered excellent conducive environment for the leaf litter decomposition and the thriving of the macro-invertebrates. It was also assumed that leaf decomposition influenced leaf toughness therefore making it an important measure in leaf toughness determination.

3.5 Limitation of the study


It was difficult to set the strings with the mesh bags in the river water and during retrieval it was difficult to get them especially after it had rained. Some mesh bags from day 16 and day 32 got lost.

CHAPTER FOUR

4.0 Results


Table 1 Physico-chemical parameters measured on each sampling date

Date (2009)

Temperature (0C)

Dissolved Oxygen (mg/l)

Dissolved Oxygen (%saturation)

Conductivity (µs cm-1)

pH

13th November

19.15

2.22

24.1

206

7.8

16th November

19.15

2.77

31.2

204

8.1

20th November

19.83

5.65

63.7

178

7.1

28th November

15.78

3.51

35.3

163

8.2

14th December

15.82

3.09

30.9

163

8.3

All parameters were recorded in the morning and the temperatures are probably close to the daily means. Water temperatures were higher on 13th 16th and 20th November then reduced on 28th November and 14th December. Dissolved Oxygen was highest on 20th November while pH was highest on 28th November and 14th December.



Figure ii: Rate of leaf toughness for Syzygium cordatum as determined by Penetrometer at diameter 0.79mm and 1.55mm

The trend of leaf toughness was the same for both diameters.





Figure iii: Rate of leaf toughness for Dombeya goetzenii as determined by Penetrometer at diameter 0.79mm and 1.55mm

The trend was the same for both diameters but with less weight compared to Syzygium cordatum





Figure iv: Rate of leaf toughness for Vanguera madagascariensis as determined by Penetrometer at diameter 0.79mm and 1.55mm

Beyond 17 days there was no change in leaf toughness





Figure v: Rate of leaf toughness for Rhus natalensis as determined by Penetrometer at diameter 0.79mm and 1.55mm

The trend of leaf toughness was the same for both diameters.



Table 2 Analysis of one way ANOVA







Sum of Squares

df

Mean Square

F

Sig.

Dombeya

Between Groups

4656.667

5

931.333

77.209

.000*

Within Groups

289.500

24

12.063







Total

4946.167

29










Vanguera

Between Groups

2350.429

5

470.086

5.226

.003*

Within Groups

1979.000

22

89.955







Total

4329.429

27










Rhus

Between Groups

70832.500

5

14166.500

40.487

.000*

Within Groups

8747.500

25

349.900







Total

79580.000

30










Syzigium

Between Groups

125078.152

5

25015.630

23.982

.000*

Within Groups

27120.567

26

1043.099







Total

152198.719

31










*-P<0.05 Significant Difference

There is a significant difference between the leaf toughness among the four leaf species with Vanguera and Dombeya being the softest while Rhus and Syzigium being the toughest.






Figure vi: Comparison of changes in leaf toughness with time of exposure for Dombeya, Vanguera, Rhus and Syzigium as determined by Penetrometer at diameter 0.79mm

Among the four leaf species using the 0.79mm diameter Syzigium was the toughest followed by Rhus then Dombeya. Vanguera was the least tough species. In day 16 Dombeya and Vanguera had their leaf toughness at 0.





Figure vii: Shows correlation between numbers of organisms and leaf toughness of Syzigium as determined by diameter 0.79mm

In Syzigium there was a weak positve correlation between numbers of organisms and leaf toughness and r = √0.364





Figure viii: Shows correlation between numbers of organisms and leaf toughness of Rhus as determined by diameter 0.79mm

In Rhus there was a weak positve correlation between numbers of organisms and leaf toughness and r = √0.372





Figure ix: Shows correlation between numbers of organisms and leaf toughness of Dombeya as determined by diameter 0.79mm

In Dombeya there was a positve correlation between numbers of organisms and leaf toughness and r = √0.481





Figure x: Shows correlation between numbers of organisms and leaf toughness of Vanguera as determined by diameter 0.79mm

In Vanguera there was a positve correlation between numbers of organisms and leaf toughness and r = √0.613





Figure xi: Shows correlation between dry weight and leaf toughness of Vanguera as determined by diameter 0.79mm

In Vanguera there was a strong positve correlation between dry weight and leaf toughness and r = √0.813





Figure xii: Shows correlation between dry weight and leaf toughness of Dombeya as determined by diameter 0.79mm

In Dombeya there was a strong positve correlation between dry weight and leaf toughness and r = √0.846





Figure xiii: Shows correlation between dry weight and leaf toughness of Rhus as determined by diameter 0.79mm

In Rhus there was a strong positve correlation between dry weight and leaf toughness and r = √0.868





Figure xiv: Shows correlation between dry weight and leaf toughness of Syzigium as determined by diameter 0.79mm

In Syzigium there was a strong positve correlation between dry weight and leaf toughness and r = √0.957 was the highest value among the four species


CHAPTER FIVE

5.0 Discussions of the results

5.1 River Njoro Physico-chemical parameters


The values of physic-chemical parameters recorded (Table 2) have deviated a lot from those recorded by the long term study of Magana (2000) between January 1998 and June 1999. The temperatures have increased to about an average of 17 0C, Dissolved Oxygen in %saturation has reduced to an average of 40, conductivity and pH have also increased. These changes were attributed to the changing climatic conditions around Njoro which was experiencing a continuous dry spell.

5.2 Comparison of leaf toughness among the four leaf species


Breakdown rates of Dombeya goetzenii, Syzygium cordatum, Rhus natalensis and Vanguera madagascariensis (Figure VI) in the tropical second order stream River Njoro were faster than those reported for other leaf species in temperate (Ostrofsky, 1997) and tropical African and American streams (Mathuriau & Chauvet, 2002). This study has clearly shown that Dombeya goetzenii and Vanguera madagascariensis were the softest due to their fast break down rates (they had disappeared by day 16, Figure VI) Syzygium cordatum had the lowest breakdown rate and therefore the toughest among the four leaf species. The slow rates of Syzygium cordatum and Rhus natalensis could be related mainly to two interdependent factors: intrinsic leaf characteristics and environmental/climate factors. A first factor could be the physical and chemical properties of S. cordatum and R. natalensis The leaf nutrient content, especially nitrogen, has been shown to be a good predictor of breakdown rates, with a fast breakdown of nitrogen-rich leaves like the ones for Dombeya goetzenii and Vanguera madagascariensis.

Lignin can also be used to predict breakdown. Leaves of S. cordatum and R. natalensis percentages of lignin, which is in the upper range of values reported by Gessner (2005). Lignin is a structural constituent conferring toughness to leaves, protecting them from herbivory and microbial infections, and providing waterproofing properties to plant cell walls; it is particularly difficult to biodegrade and limits the degradation of other plant cell compounds. The high lignin content in S. cordatum and R. natalensis could be a consequence of the water and nutrient stress of the River Njoro and may explain the reported low herbivory in this system (Dobson et al 2002).

Leaf toughness is therefore an explanation for the differences in breakdown rates observed in leaves of Dombeya goetzenii, Syzygium cordatum, Rhus natalensis and Vanguera madagascariensis in the tropical stream River Njoro and this highly explains the strong positive correlations between dry weight and leaf toughness of the four leaf species (r = √0.813, √0.846, √0.868, and √0.957 for Vanguera, Dombeya, Rhus and Syzigium) as shown by figures XI, XII, XIII and IX

5.3 Correlation between numbers of organisms and leaf toughness among the four species


S. cordatum and R. natalensis were poorly colonized by invertebrates where r = √0.364 and √0.372 respectively (Figures VII & VIII). However mining Chironomids were observed feeding on decomposing leaves of D. goetzenii and V. madagascariensis and accelerating their rate of litter breakdown therefore reducing their leaf toughness where r = √0.481 and √0.613 respectively (Figures IX & X) Shredding invertebrates at River Njoro normally occur at much lower densities, suggesting a qualitative difference between streams in temperate and tropical regions. The reason for the paucity of shredders is unclear, as tropical stream channels often contain plenty of potential food resources for this guild. For example, Dobson et al (2002) demonstrated high standing stock of detritus in several Kenyan highland streams and proposed a series of possible explanations for the absence or low density of shredders.

One of these related to leaf litter quality; they speculated that riparian trees in Kenya produce leaf litter that is much more refractory and therefore of considerably lower nutritional value than that produced by riparian trees in the North Temperate Zone. This proposal was based upon little more than a cursory examination of leaves from the dominant riparian trees, but it was supported by Mathooko et al (2002c), who determined a slow decay rate for leaves of the common riparian tree Syzygium cordatum.

Shredder invertebrates are apparently scarce, probably caused by low quality of food resources, reinforced by the spates washing the leaf litter from the stream-bed. Breakdown rates are therefore very slow and catalyzed mainly by microbial activity. Physical abrasion may also be important during the rainy season, but this factor was not evaluated since the study was carried out during the dry season.

CHAPTER SIX

6.0 Conclusions and Recommendations

6.1 Conclusions


There is a significant difference in leaf toughness among the four leaf species where the toughest were Syzygium cordatum and Rhus natalensis. Dombeya goetzenii and Vanguera madagascariensis proved to be the least tough. The decomposition rates found are fast according to the boundaries proposed by Petersen and Cummins (1974). These ranges fall within others recorded elsewhere in the tropics. Despite the apparent absence of shredders in the River Njoro mass loss is mediated by microbial and physical factors. Flows in the River Njoro remained low throughout the study with little evidence of severe abrasion and can therefore be concluded that microbial activity was the main reason that led to a decrease in the leaf toughness as it was the main processing agent. Rapid decomposition rates are often associated with detritus that is of a high quality food resource for shredders but the implication from these results is that paucity of shredders in the River Njoro cannot be explained in terms of poor detritus quality.

The strong correlations between dry weight and leaf toughness among the four tree species is a strong base to conclude that indeed leaf decomposition is a measure of leaf toughness.



6.2 Recommendations


A related study needs to be carried out during a rainy season in order to compare the differences in the leaf toughness, Furthermore many other topics related to litter breakdown and leaf toughness still need to be investigated in River Njoro, including the life history of true shredders, the capability of other invertebrates to feed on decomposing leaves, the presence and abundance of mining shredders the variability of leaf litter input to streams.

References


Allan, J.D. (1995): Stream ecology: Structure and function of running waters. Chapman & Hall, London. 388 pp

Cao, Y., A.W. Bark, & W.P. Williams, (1996): Measuring the responses of macroinverterbrate communities to water pollution: A comparison of multivariate approaches, biotic and diversity indices. Hydrobiologia, 341: 1-9

Dobson, M., A. Magana, J.M. Mathooko & F.K. Ndegwa (2002): Detritivores in Kenyan highland streams: more evidence for the paucity of shredders in the tropics? Freshwater Biology 47: 909-919

Dudgeon, D. (1982): An investigation of physical and biological processing of two species of leaf litter in Tai Po Kau Forest Stream, New Territories, Hong Kong. Hydrobiologia 96: 1-32

Flindt M.R. & Lillebø A.I. (2005): Determination of Total Nitrogen and Phosphorus. In: Methods to study litter decomposition: A practical guide (Eds M.A.S. Graca, F. Bärlocher & M.O. Gessner), pp. 53-59. Springer, Dordrecht.

Gessner M.O. (2005): Ergosterol as ameasure of fungal biomass. . In: Methods to study litter decomposition: A practical guide (Eds M.A.S. Graca, F. Bärlocher & M.O. Gessner), pp. 189-195. . Springer-Verlag, Dordrecht.

Goncalves, J.F. Jr, Graca, M.A.S. & Callisto M. (2007): Litter decomposition in a Cerrado savannah stream is retarded by leaf toughness, low dissolved nutrients and a low density of shredders. Freshwater biology, 52: 1440-1451

Goncalves, J.F. Jr, Graca, M.A.S. & Callisto M. (2006): Litter breakdown dynamics at three streams in temperate, Mediterranean and tropical Cerrado climates. Journal of the North American Benthological Society, 25: 344-355

Hutchens J.J. Jr & Wallace B. (2002): Ecosystem linkages between Southern Appalachian headwater streams and their banks: Leaf litter breakdown and invertebrate assemblages. Ecosystems, 6: 80-91

Kar, J.R. & E.W. Chu, (1998): Restoring life in running waters: Better biological monitoring. Island press, 220pp

Maamri A., Chergui H. & Pattee E. (1997): Leaf litter processing in a temporary in a temporary northeastern Morrocan river. Archiv für Hydrobiologie, 140: 513-531

Magana A.E.M. (2000): Inputs and retention of particulate organic matter (CPOM) at in a tropical stream, River Njoro, Kenya. Ph.D Thesis, University of Vienna, 135pp

Mathooko J.M. (1995): The retention of plant Course Particulate Organic Matter (CPOM) at the surface of the wet store and dry store zone of the River Njoro, Kenya, African journal of ecology, 33: 151-159

Mathooko J.M. & S.T. Kariuki, (2000a): Disturbances and species distribution of the riparian vegetation of a Rift Valley stream. African journal of ecology, 38: 123-129

Mathooko, J.M., C.M. M’Erimba & M. Leichtfried (2000b): Decomposition of leaf litter of Dombeya goetzenii in the River Njoro, Kenya. Hydrobiologia 418: 9-19

Mathooko J.M., A.M. Magana & I. M Nyang’au (2002c): Decomposition of Syzygium cordatum leaves in a Rift Valley stream ecosystem. African journal of ecology, 38: 265-368

Mathuriau C. & Chauvet E., (2002): Breakdown of leaf litter in a neotropical stream. Journal of the North American Benthological Society, 21: 384-396

Ostrofsky M.L. (1997): Relationship between chemical characteristics of autumn shed leaves and aquatic processing rates. Journal of the North American Benthological Society, 16: 750-759



Petersen R.C. & Cummins K.W., (1974): Leaf processing in a woodland stream. Freshwater biology, 4

APPENDICES


Appendix 1: Procedure and Time frame.

Month/activity

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Preparation of proposal




























Data collection




























Data analysis and report writing




























Finalizing report




























Final report submission
































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