Planktonic communities as indicators of water quality in mangrove lagoons; a Jamaican case study

Abstract

1 - In mangrove lagoons, where natural conditions of high turbidity, detritus and microbiological activity combined with low light render traditional water quality indices unreliable, the planktonic community is proposed as a more appropriate and accurate index of eutrophication status. 2 - Over a 12 month period, monthly sampling was conducted at 9 stations representing different degrees of eutrophication. 3 - Phytoplankton size fractionated biomass, species composition and abundance as well as zooplankton total abundance, species composition and abundances of selected groups were examined along with depth, temperature, salinity, dissolved oxygen, pH, particulate organic matter, and reduction / oxidation potential (redox). 4 - Phytoplankton were collected using a horizontal niskin whole water sampler, zooplankton hauls (vertical/oblique) were taken using a modified Wildco net (158 &µ956;m mesh aperture 0.2 m hoop diameter) and a Hydrolab Surveyor IV Water Quality Data System was used to collect physic-chemical data in situ. 5 - While dissolved oxygen, salinity and pH demonstrated potential for use as water quality indicators, particulate organic matter, redox and temperature demonstrated inconclusive variations or insignificant differences between lagoons of known water quality. 6 - Phytoplankton biomass (Chlorophyll a) was among the most reliable planktonic indices distinguishing four groups of stations with different water qualities: oligotrophic (0.21-0.55 mg m^-3) mesotrophic (0.57 – 2.55 mg m^-3) eutrophic (3.00 to 6.55 mg m^-3) and extremely eutrophic (>31.17 mg m^-3). 7 - This was attributed to diatom and dinoflagellate cells of differing sizes being able to characterise eutrophication even after the source of eutrophication had dissipated and while physico-chemical parameters remained unchanged. 8 - Mean zooplankton abundances were also reliable, distinguishing pristine stations (789 - 3,111 animals m^-3) and highly eutrophic stations (114,970 – 563,339 animals m^-3). 9 - The group ‘Larvae’ and the copepod Dioithona occulata were major contributors to mean abundances, since as small herbivores they are able to take advantage of high phytoplankton concentrations. 10 - Other taxa like Acartia tonsa and harpacticoid copepods were ubiquitous and although never numerically dominant showed high numbers at the more eutrophic sites. 11 - This makes relative abundances of cosmopolitan species as reliable an index as presence absence of key species in mangrove lagoons.