Temporal variations in phytoplankton, particulates, and colored dissolved organic material based on optical properties during a Long Island brown tide compared to an adjacent embayment

Abstract

Since 1985, the coastal embayments of Long Island, New York, have been plagued with recurrent blooms, aptly called brown tides, of the pelagophyte Aureococcus anophagefferens. The distinct ocean color observed during these blooms suggests that optical methods can be used as a tool to study, detect, and track brown tides. Thus, the goal of our project was to compare the optical properties and pigment composition during bloom and non-bloom conditions and assess temporal variations in the phytoplankton and other constituents in the seawater associated with bloom development. From 17 May to 8 June 2000, we measured a time series of particle size distributions and concentrations as well as size-fractioned algal pigments and optical properties in two Long Island embayments where brown tides are known to occur. During our study, A. anophagefferens represented an insignificant contribution to the algal community in West Neck Bay (WNB), whereas a bloom developed in Quantuck Bay (QB). Initially, temperature and salinity were similar at the two locations; however, bulk optical properties, chlorophyll, and particle concentrations were nearly a factor of 2 greater at QB. Bulk optical properties remained constant at WNB, yet increased exponentially at QB as the bloom developed. The composition of particulates, including phytoplankton, varied little at QB, and the optical properties suggested the dominance of A. anophagefferens (confirmed by microscopy). The largest temporal variations were observed in the colored dissolved organic material (CDOM); the colloidal (0.2–0.7 μm) fraction, exhibiting a strong protein-like signal, increased dramatically at the height of the bloom. At WNB particle sizes and algal composition varied despite the invariant bulk optical properties; CDOM variations were minimal. Overall, the optical properties in the two bays demonstrated that at QB temporal variations were dominated by biomass and colloidal protein changes, whereas shifts in the algal community occurred at WNB. This study demonstrates the utility of in situ optical observations to resolve temporal changes in the ecological conditions associated with algal bloom development.