Oceanographic features and zooplankton community structure at Mayaguez Bay, Puerto Rico

This study was part of a multidisciplinary research effort aimed at establishing a characterization of the oceanographic features and the zooplankton community structure that could help to evaluate the response of marine communities to a water quality restoration initiative. Monthly samplings consisted of conductivity, temperature and depth (CTD) and chlorophyll-a fluorescence profiles. Grab samples for nutrients determination and Secchi disc measurements. Zooplankton was collected by oblique tows using a 1m2 TuckerTrawl System with three 0.202mm mesh nets. Density profiles at the Ocean station showed permanent stratification with well-developed pycnoclines associated with increasing salinity and declines of water temperature with depth. Inshore stations had well-mixed water columns, with vertical stratification associated to freshwater lenses at the surface during heavy rainfall events. Maximum water column chlorophyll-a concentrations were consistently higher at Atuneras and AAA stations. Chlorophyll-a concentration was positively correlated with streamflow at Manchas and Rodríguez, but varied independently from the streamflow at the other stations. Organic nitrogen (Org-N) was the nutrient in highest concentration, followed by ammonium-ion (NH₄+) and nitrate (NO₃− -N). Concentrations of orthophosphate (PO₄−3 -P) were only detected during April in Añasco. These data suggest that phosphate may be regulating phytoplankton production at stations with good light penetration. Secchi disk readings penetrated less than 50% of the water column at Atuneras and AAA stations.; Total zooplankton abundance varied between 828–2,135 ind m −3 during the dry season and between 1,365–3,040 ind m −3 during the rainy season. Zooplankton abundance was higher at inshore stations (1,905–2,588 ind m−3) compared to Ocean station (1,097 ind m−3). Holoplankton represented more than 95% of total zooplankton abundance. Twenty-nine genera (19 calanoids, 7 cyclopoids, and 3 harpaticoids) were identified during the study. Principal component analysis indicated that zooplankton abundance at Ocean station explained 83% of the variation of zooplankton taxonomic groups. The higher abundances of ctenophores, cnidarians and harpacticoid copepods at Atuneras and AAA stations accounted for the separation of inshore stations suggesting that anthropogenic nutrient inputs promote scale mosaics of zooplankton taxonomic structure in the bay. Oithona spp., Corycaeus spp., and Euterpina acutifrons appear to be potentially good indicators of organic pollution for tropical systems.