Planktonic food webs in the Bangpakong estuary

Abstract

Planktonic food webs in the Bangpakong estuary of Chachoengsao Province, Thailand were investigated for the structure, abundance and biomass of plankton community, the role of pico– and nanoplankton in relation to food web and ecological efficiency of carbon-energy flow. Field samplings were conducted from eight stations in dry season and wet season of 2004 and 2007. Water quality parameters of the Bangpakong estuary were in the range of natural coastal water quality standards throughout the study except the low dissolved oxygen that indicated the deteriorated condition in the innermost of the upper estuary (St 1 and St 2). Seasonal variations play an important role in the spatial and temporal variations of salinity and nutrients. Plankton community in the Bangpakong estuary composed of planktonic organisms of different sizes ranged from picoplankton to mesoplankton. Cyanobacteria were the most abundance phytoplankton in all size classes together with phytoflagellates in nanophytoplankton population and diatoms in microplankton size class. Nano- and microplanktonic heterotrophs were dominated by ciliates, while copepods dominated the microzooplankton and mesozooplankton communities. Seasonal variability in salinity and nutrients plays an important role in shaping phytoplankton community structure. Temporal variations in phytoplankton communities were observed with diatoms and nanophytoflagellates as dominant plankton in dry season while cyanobacteria dominated the communities in wet season. Spatial variations in phytoplankton communities were observed with diatoms as dominant plankton in upper estuary while dinoflagellates dominated the communities in lower estuary. Nanophytoplankton chlorophyll_a (10.3 – 172.5 mgC.m̄³), microzooplankton (29.8 – 837.6 mgC.m̄³) and bacteria (3.7 – 384.2 mgC.m̄³) were the major constituents in plankton communities in Bangpakong estuary. Spatial and temporal of chlorophyll_a concentrations were observed with high values in wet season and upper estuary. The planktonic food web in the Bangpakong estuary consisted of four trophic levels with primary carbon food sources may come from phytoplankton. The low δ ¹³C values of zooplankton in upper estuary represented the major allochthonous primary carbon sources, while the high δ ¹³C values of zooplankton in lower estuary represented the major autochthonous primary carbon sources. Food web model derived from this study was in agreement with those reported in the past. Phytoplankton and bacteria biomass were a primary food source of many compartments in the estuarine food web. Copepods, the most prominent heterotrophic plankton in terms of biomass, acted as a link from producers to other consumers in higher trophic levels. The transfer efficiency from primary producers to consumers was lower than from detritus, indicating that the Bangpakong estuary was more dependent on the detritus than primary producers to generate total system throughput. Total system throughput (TST) in the Bangpakong estuary ranged between 14,625 – 72,988 mgC. m̄³.yr̄¹ indicating high productivity of this ecosystem. The low conservation of organic matter (FMPL index) in ranged 0.53 – 13.28 and low fraction of recycled matter (the FCI index) in ranged 2.78 – 4.05 show that the Bangpakong estuary was a relatively immature ecosystem. Net ecosystem metabolism also indicated that the ecosystem of Bangpakong estuary behaved as autotrophic system in the dry season but became the hetrotrophic one in the wet season. This food web model may be useful for the construction of a mass-balance ecosystem model or a time-dynamics simulation model in order to simulate the impact of changing environment such as salinity and nutrients on fisheries resources. The knowledge of the planktonic food web in Bangpakong estuary is a step towards a better understanding of production processes in coastal ecosystem of Thailand.