Vertical migration patterns of phytoflagellates in relation to light and nutrient availability in a shallow microtidal estuary

Abstract

Physical and chemical vertical gradients in estuaries are often steep, with changes on the scale of cm to m strongly affecting a phytoplankter’s exposure to limiting nutrients and light. Two diel field studies were conducted during June and July 2001 to establish how the composition and productivity of the phytoplankton are influenced by the vertical water column structure within the shallow, eutrophic, microtidal Neuse River Estuary, North Carolina, USA. During both studies, an upper photic mixed layer with low dissolved inorganic nitrogen (DIN) (~ 1 μM) lay above a sub-halocline, aphotic region with elevated DIN (~2 to 5 μM). Phytoflagellates were dominant, and observed diel vertical migration (DVM) patterns are likely an important reason for their success in this N-limited system characterized by strong vertical separation of light and DIN resources. Integrated water column primary productivities during June and July were 15 and 113% greater, respectively, than expected for a vertically homogenous phytoplankton community. The high degree of productivity enhancement in July was due to daytime near-surface aggregation in response to low-light, overcast conditions. Displacements of flagellate populations during DVM were positively correlated with cell size, exposing larger cells to higher light and nutrient levels over the diel period. Since larger phytoplankton are generally less efficient at utilizing scarce resources, such vertical niche partitioning according to cell size should enhance community-level productivity. Interspecific variation in vertical migration patterns is an important trait of phytoplankton that promotes complementarity of resource utilization and contributes to the relationship between diversity and productivity within phytoplankton communities.