Modeling Phytoplankton Community Changes in the Cariaco Basin, Venezuela

Marine phytoplankton communities are affected by environmental changes and anthropogenic influences with consequences to marine food webs. To study the long-term effects of environmental drivers, we have relied on time-series observations at particular locations of the world ocean. One such location is the Carbon Retention in a Colored Ocean (CARIACO) time-series station located in the Cariaco Basin off the coast of Venezuela. Previous studies using high-performance liquid chromatography (HPLC) and microscopy data have detected a shift in phytoplankton community composition between two measurement regimes. The first regime lasted from January 1996 to October 2000, and the second from July 2006 to December 2010. Observed changes between the regimes included diminished phytoplankton bloom intensities in the latter period, coinciding with increased zooplankton densities and changes in the phytoplankton community structure towards smaller taxa. We present here an ecosystem model resolving multiple phytoplankton functional types to investigate the environmental drivers of changes in the phytoplankton community of the Cariaco Basin. Model results faithfully recreate the annually aggregated dynamics of the environmental and ecological variables measured during the first regime. By varying the relative magnitudes of (1) higher-order predation on zooplankton and (2) mixing intensity, we show that both top-down and bottom-up effects can explain the observed community shifts with our model. Our study has particular relevance for understanding how coastal ecosystems like the Cariaco Basin can reorganize under the influence of multiple environmental drivers and how these changes relate to the maintenance of ecosystem functions.