Characterization of phytoplankton variability in the Cariaco Basin using spectral absorption, taxonomic and pigment data

Abstract

The spectral absorption coefficient of marine phytoplankton provides information on phytoplankton community structure, biomass, and general physiological conditions. These variables are necessary for understanding and predicting ocean productivity, carbon fluxes, underwater light propagation, water quality, and for assessing marine photochemical processes. The Cariaco Basin, located on the continental shelf of Venezuela in the southeastern Caribbean Sea, is the site of the CARIACO Ocean Time-Series project. Since 1995, CARIACO has collected bio-optical (hyperspectral inherent and apparent optical properties — IOPs and AOPs, respectively), biogeochemical and ecological observations to characterize local ecosystem variations in response to regional and global changes in climate. We examine phytoplankton taxonomic and pigment time series data collected by this program between 2006 and 2012 to understand how seasonal changes in these parameters relate to bio-optical data (i.e., absorption spectra). TChla and accessory pigments varied seasonally in response to changes in the phytoplankton community composition, with higher concentrations of microphytoplankton (>20μm; 45%) during upwelling (December–April) than during the rainy season (16%; May/June–October/November). Picophytoplankton (<2μm) dominated during the rainy season (66%). The absorption properties also exhibited seasonal variations. Diagnostic pigments could not be identified in a quantitative way using derivative analysis of phytoplankton absorption, likely because of overlapping of absorption spectra among the pigments present. The POC:TChla ratio at CARIACO was variable and dependent on bulk carbon (not necessarily related to phytoplankton) and the functional groups present at any given time, underscoring the fact that using a fixed ratio of POC:Chla in biogeochemical models can lead to large uncertainties in carbon budgets from coastal zones. Low POC:TChla was associated with microphytoplankton size class (diatoms), while picophytoplankton (cyanobacteria) exhibited higher ratios. These results contribute to furthering our understanding of coastal phytoplankton dynamics and how they relate to optical signatures.