Impact of zooplankton grazing on Alexandrium blooms in the offshore Gulf of Maine

Abstract

Abstract Zooplankton grazing was investigated by shipboard experiments during natural blooms of Alexandrium spp. in the offshore Gulf of Maine in spring and/or summer of 1998, 2000, and 2001. Grazing studies were done in conjunction with studies of accumulation of Alexandrium toxins in the zooplankton, as part of the ECOHAB-Gulf of Maine regional program. Several species of copepods, marine cladocerans, and appendicularians were allowed to graze upon natural phytoplankton assemblages, at ambient temperatures (14–17 °C). Grazing was measured by quantitative microscopic analyses of disappearance of phytoplankton cells in initial, control, and experimental food suspensions. Thus, we were able to examine grazing upon Alexandrium in comparison to grazing on other co-occurring phytoplankton taxa. Even during Alexandrium “blooms,” this dinoflagellate was a minor component of the overall phytoplankton assemblage. It was present at stations where grazing experiments were conducted at levels of 0.12–7.57×103 cells l−1, or 0.03–3.93% of total phytoplankton cells. Maximum ingestion of Alexandrium accounted for only up to 3.2% of total cells ingested. Phytoplankton assemblages were dominated by athecate microflagellates, and to a lesser extent by diatoms and non-toxic dinoflagellates. Microflagellates were present at abundances of 159.62–793.93 cells ml−1, or 60.6–95.56% of total cells. Grazing on microflagellates accounted for 35.59–98.21% of total grazing. Grazing on Alexandrium spp. and microflagellates was generally non-selective, with these taxa being ingested in similar proportions to their availability in food assemblages. Grazing on diatoms was selective, with diatoms being disproportionately ingested, compared to their proportions in food assemblages. There were no apparent adverse effects of Alexandrium on grazers during incubations of 18–24 h, and grazer survival was 100%. Estimated daily zooplankton grazing impact on Alexandrium spp. field populations by field populations of experimental grazers averaged 5.79% (range=0–117%). Extrapolating experimentally determined grazing rates to total zooplankton assemblages increased potential grazing impact to 0–667.77% (mean=114.7%). However, these potential impacts are likely overestimations, because toxin accumulation data indicated that many of the most-abundant zooplankters (Oithona similis copepodites and copepod nauplii) likely graze only minimally upon Alexandrium spp. Thus, antipredation effects of high concentrations of Alexandrium on some grazers reported from some laboratory studies may only occur rarely in nature, because of low individual zooplankter grazing rates on Alexandrium, and dilution of grazing upon it by grazing on other food sources such as abundant microflagellates and diatoms.