Trophic linkage: the importance of micro-algae to the fisheries of Boeung Tonle Sap, Cambodia

Previous dogma has stated that the allochthonous organic material from the seasonally inundated floodplain vegetation is a major source of carbon supporting the fisheries of Boeung Tonle Sap. However, dual stable isotopic analysis on selected and commercially importance fish species suggested that algae were their major primary carbon sources. Cirrhinus microlepis and Henicorhychus lobatus derived their carbon source almost exclusive from phytoplankton. Charma striata, Cyclbcheilichthys enoplos, and Pangasius larnaudii derived their carbon sources, via unknown primary consumers, from attached micro-algae. Barbonymus gonionotus was found to feed almost exclusively on algal scum (presumably cyanobacteria). However, micro-algae were not always the dominant carbon source for these fish - its importance varied amongst species and between the defined hydrological phases. The phytoplankton biomass quantified in this study was comparable to that in previous reports from Boeung Tonle Sap and showed similar ranges of biomass to many other inundated floodplain system in neo-tropical, Afro-tropical and Asian tropical regions. The mean of surface phytoplankton measured in the lake from nine successive phases of the Mekong flood pulse cycles fluctuated in the ranges: chlorophyll a 3-55 mg m"3, pheophytin a 1 - 40 mg m"3, and total chlorophyll a 5 - 70 mg m"3. Depthaveraged chlorophyll a varied in the ranges 4 - 55 mg m"3, pheophytin a 4 - 40 mg m"3, and total chlorophyll a 8 - 65 mg m"3. Depth-integrated chlorophyll a was in the range 20 - 60 mg m"2, pheophytin a 3 - 35 mg m"2, and total chlorophyll a 30 - 100 mg m"2. Phytoplankton biomass varied over time, and the most important temporal trend was the apparent relationship with the Mekong flood pulse cycle. Boeung Tonle Sap is subjected to large water level fluctuations, which result in seasonal exchanges of large volumes of water, increasing or diminishing the impact of the different physico-chemical factors that thought to affect phytoplankton processes. The results of the current study suggested that hydrological changes, resulting from the Mekong flood pulse cycle, did not influence phytoplankton biomass directly; it was probably the availability of nutrients and light that influenced phytoplankton in Boeung Tonle Sap. The seasonally inundated floodplain of Boeung Tonle Sap, besides providing hydraulic and structural habitats in the developmental life-cycle of fish, offers an important habitat for the proliferation of periphyton (as there is no permanent standing vegetation biomass in the lake). The periphyton biomass measured here was consistently lower than in many other seasonally inundated tropical floodplain systems and those from temperate regions. Sampling from all the sites and hydrological phases, the mean chlorophyll a, pheophytin a and total chlorophyll a were in the range of less than 1 to 13 mg m", 7 mg m"2, and 17 mg m"2 respectively. The contribution of micro-algae found within, or attached to, the lake proper benthic sediment was most likely insignificant to the overall primary productivity, as the presence of pheophytin a in the lake benthic sediment outweighed the presence of living or proliferating benthic micro-algae (measured as chlorophyll a) on both spatial and temporal scales.