Phytoplankton Response to a Stratification-Mixing Cycle in the York River Estuary during Late Summer

Abstract

As part of a larger multidisciplinary study of the lower York River estuary, phytoplankton response to a tidally related cycle of stratification-destratification was examined during August 1978. A “red water bloom” dominated by the dinoflagellate Cocchlodinium heterolobatum was initially observed in the lower York River coincident with the spring tide-induced water column destratification event. It is proposed that the dinoflagellates initiating the red tide were advected into the estuary in deep water during the preceding period of stratification or were derived from cysts in the sediments and that destratification provided access to the surface waters. The extent of the red water increased during the ensuing restratified period in the York River, and several lines of evidence indicated that C. heterolobatum migrated diurnally between ammonium enriched waters below the halocline (8–10 m) and the relatively nutrient-poor surface waters. Other estuarine systems in which phytoplankton blooms associated with alternating periods of stratification-destratification have been observed are noted. The results illustrate the close relationship between phytoplankton and hydrographic dynamics in this estuarine system and emphasize the necessity to include the study of hydrographic processes in the study of phytoplankton dynamics.