Influence of daylight surface aggregation behavior on nutrient cycling during a Karenia brevis (Davis) G. Hansen & Ø. Moestrup bloom: Migration to the surface as a nutrient acquisition strategy

The toxic HAB dinoflagellate Karenia brevis (Davis) G. Hansen & Ø. Moestrup (formerly Gymnodinium breve) exhibits a migratory pattern atypical of dinoflagellates: cells concentrate in a narrow (∼0-5 cm) band at the water surface during daylight hours due to phototactic and negative geotactic responses, then disperse downward at night via non-tactic, random swimming. The hypothesis that this daylight surface aggregation behavior significantly influences bacterial and algal productivity and nutrient cycling within blooms was tested during a large, high biomass (chlorophyll a >19 μg L−1) K. brevis bloom in October of 2001 by examining the effects of this surface layer aggregation on inorganic and organic nutrient concentrations, cellular nitrogen uptake, primary and bacterial productivity and the stable isotopic signature (δ15N, δ13C) of particulate material. During daylight hours, concentrations of K. brevis and chlorophyll a in the 0-5 cm surface layer were enhanced by 131% (±241%) and 32.1% (±86.1%) respectively compared with an integrated water sample collection over a 0-1 m depth. Inorganic (NH4, NO3+2, PO4, SiO4) and organic (DOP, DON) nutrient concentrations were also elevated within the surface layer as was both bacterial and primary productivity. Uptake of nitrogen (NH4+, NO3−, urea, dissolved primary amines, glutamine and alanine) compounds by K. brevis was greatest in the surface layer for all compounds tested, with the greatest enhancement evident in urea uptake rates, from 0.08 × 10−5 ng N K. brevis cell−1 h−1 to 3.1 × 10−5 ng N K. brevis cell−1 h−1. These data suggests that this surface aggregation layer is not only an area of concentrated cells within K. brevis blooms, but also an area of increased biological activity and nutrient cycling, especially of nitrogen. Additionally, the classic dinoflagellate migration paradigm of a downward migration for access to elevated NO3− concentrations during the dark period may not apply to certain dinoflagellates such as K. brevis in oligotrophic nearshore areas with no significant nitricline. For these dinoflagellates, concentration within a narrow surface layer in blooms during daylight hours may enhance nutrient supply through biological cycling and photochemical nutrient regeneration.