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Macrobenthic Community Structure Across an Inter- and Subtidal Gradient in a Mangrove Estuary

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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by South East Academic Libraries System (SEALS) Macrobenthic community structure across an inter- and subtidal gradient in a mangrove estuary By Christoff J. Groenewald Submitted to the Department of Zoology in fulfillment of the requirements for the Degree of Magister Scientiae in the Faculty of Science at the Nelson Mandela Metropolitan University Supervisor: Prof. T.H. Wooldridge Co-Supervisor: Dr. P.E.D. Winter Submitted: January 2010 1 Acknowledgements I would like to greatly acknowledge my supervisor Prof. Tris Wooldridge and co-supervisor Dr Deo Winter for their guidance, patience and support throughout the course of this project. I am extremely grateful to the National Research Foundation (NRF), who supplied me with financial assistance for the duration of the research, as well as the Nelson Mandela Metropolitan University (NMMU) who also supplied funding for the research. I would like to thank the technical and support staff of the Zoology and Geology Departments at NMMU. The following persons contributed greatly towards the completion of this study: staff of the Mngazana guest farm and uMngazi River Bungalows, Brian Seale, Neil Skillicorn, Shaun Deyzel, Mathys Vosloo, Jesse Jegels, Karien Bezuidenhout, Caroline Pringle, Dr Isabelle Papadopoulos, Ryan Wasserman and Eurika Ramdin. Finally, I would like to thank my Parents and friends for their constant support, patience and encouragement. i Abstract Macrozoobenthic community structure and composition was investigated along a subtidal- intertidal gradient in the Mngazana Estuary. Six transects were sampled between the spring high water mark (HWST) and the bottom of the river channel in the lower estuary. Fifteen replicate samples were collected along each transect using a Van Veen type grab (211 cm2 bite) during each of three sampling sessions. Samples were sieved through a 500 µm mesh bag and the invertebrates stored in bottles for further analysis in the laboratory. Additional grab samples were collected for sediment particle size analysis and organic matter. Physical variables measured at each transect included: salinity, temperature, dissolved oxygen, depth, pH, percentage mud, organic content and turbidity. Sediment compactness was measured at all intertidal transects and additional sediment samples were collected at mid shore and high shore transects for percentage water content analysis. A total of 104 species were recorded along the intertidal-subtidal gradient in the sampling area. Species richness was higher in the subtidal zone compared to the intertidal zone and polychaetes numerically dominated the macrozoobenthic community at most transects, during all three sessions. At high shore transects the community was characterised by having fewer species, consisting mostly of brachyurans, polychaetes and gastropods. Shannon diversity index (H’) was generally higher for subtidal transects (¯x = 2.3; range: 2.8 to 1) than for intertidal transects (¯x = 1.4; range: 2.2 to 0.6) indicating that the distribution of individuals among species in the intertidal zone experienced greater variability. Results for Hill’s numbers followed the same trend as Shannon diversity with subtidal communities mostly consisting of abundant species followed by very abundant species. Intertidal communities generally exhibited lower numbers of abundant and very abundant species. Sedimentary characteristics played a major role in structuring benthic communities in comparison to other physico-chemical variables. Organic content and mud content of the substrate were identified as important factors influencing community patterns observed along the subtidal-intertidal gradient. In addition, sediment compactness and water content of the substrate was found to influence intertidal community structure. Subtidal community structure possibly had a greater dependence on seasonal variations in abiotic and/or biotic factors. Cluster dendrograms used in conjunction with MDS ordination mapping revealed that macrozoobenthic communities were generally distinct between high shore intertidal transects ii and subtidal transects. Most species exhibited a broad spatial distribution along the subtidal- intertidal gradient with mid and high shore transects being the exception. Most species also exhibited marked shifts in abundance and this was especially noticeable at the transition between the subtidal and intertidal zone. Two polychaete species, Prionospio sexoculata and Capitella capitata, were very abundant species and featured amongst the most numerically dominant species collected during each sampling session. Key words: subtidal, intertidal, macrozoobenthos, community structure, environmental variables, sedimentary characteristics. iii Table of Contents Page Chapter 1. ............................................................................................................ 1 Introduction .......................................................................................................... 1 1.1. Sediment characteristics ......................................................................... 2 1.2. Salinity ..................................................................................................... 3 1.3. Tidal inundation ....................................................................................... 4 1.4. Biological interactions as factors influencing benthic communities .......... 6 1.4.1. Aquatic vegetation ......................................................................... 6 1.4.2. Bioturbation ................................................................................... 7 1.4.3. Predation ....................................................................................... 8 1.4.4. Recruitment ................................................................................... 9 1.5. Motivation for benthic studies ................................................................ 10 1.6. Previous benthic research on the Mngazana Estuary ........................... 11 1.7. Purpose of the study ............................................................................. 12 Chapter 2. .......................................................................................................... 13 Description of the estuary and study area .......................................................... 13 2.1. Location and physiography.................................................................... 13 2.2. Study site ............................................................................................... 14 Chapter 3. .......................................................................................................... 16 Materials and Methods ....................................................................................... 16 3.1. Field sampling procedure ...................................................................... 16 3.1.1. Experimental design .................................................................... 16 3.1.2. Biological sampling ...................................................................... 17 3.1.3. Physico-chemical sampling ......................................................... 18 3.2. Laboratory procedures .......................................................................... 18 3.3. Data analysis ......................................................................................... 19 3.3.1. Environmental data analysis ........................................................ 20 3.3.2. Community structure ................................................................... 20 3.3.3. Multivariate species analysis ....................................................... 21 3.3.4. SIMPROF .................................................................................... 22 3.3.5. Connecting community structure and environmental data .......... 22 3.3.6. Similarity breakdown and species analysis ................................. 22 Chapter 4. .......................................................................................................... 24 Results ............................................................................................................... 24 4.1. Physico-chemical variables ................................................................... 24 4.1.1. Salinity ......................................................................................... 24 4.1.2. Temperature ................................................................................ 24 4.1.3. Dissolved oxygen ........................................................................ 25 iv 4.1.4. Mud and silt fraction .................................................................... 25 4.1.5. Organic matter ............................................................................. 25 4.1.6. Sediment water content ............................................................... 26 4.1.7. Compactness of the sediment ..................................................... 26 4.2. Multivariate analysis of environmental data ........................................... 29 4.2.1. Physico-chemical analyses ......................................................... 29 4.2.2. Particle size analysis ................................................................... 32 4.2.3. Sedimentary characteristics of intertidal transects ...................... 35 4.3. Biological analysis ................................................................................
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