Search

Zooplankton grazing is often a significant loss term for phytoplankton populations, including harmful algae, impacting the development and decline of blooms. However, detecting and quantifying predation on phytoplankton is often challenging, particularly during early bloom stages when phytoplankton cell concentrations are low. In this study, we used polymerase chain reaction (PCR) to detect ingestion of toxic dinoflagellates of the Alexandrium tamarense species complex by two copepods, Acartia hudsonica (laboratory population) and Calanus finmarchicus (field population). Recent ingestion of Alexandrium fundyense cells was indicated by positive amplification of an LSU rDNA fragment specific to A. fundyense from whole copepod extracts. In laboratory experiments, A. fundyense DNA was detectable for 2―4 h post-ingestion in A. hudsonica fed A. fundyense, but not detected in animals fed other phytoplankton, or starved. In field samples, ingestion of A. fundyense by C. finmarchicus was confirmed by PCR, including at four stations where the A. fundyense concentration was ≤ 14 cells L ―1 . At these low prey concentrations, ingestion rates on A. fundyense may have been as low as 1 cell copepod ―1 day ―1 . Nevertheless, simulations of A. fundyense population growth suggest that a few predators L ―1 have the potential to curb the early development of a slow-growing bloom, even if ingestion rates are extremely low. Low predation rates can still have a large impact when prey populations are small.

A combination of observations and model results suggest a mechanism by which coastal blooms of the toxic dinoflagellate Alexandrium fundyense can be initiated from dormant cysts located in offshore sediments. The mechanism arises from the joint effects of organism behavior and the wind-driven response of a surface-trapped plume of fresh water originating from riverine sources. During upwelling-favorable winds, the plume thins vertically and extends offshore; downwelling winds thicken the plume and confine it to the nearshore region. In the western Gulf of Maine, the offshore extent of the river plume during upwelling conditions is sufficient to entrain upward-swimming A. fundyense cells germinated from offshore cyst beds. Subsequent downwelling conditions then transport those populations towards the coast.

Abst ract. Surveys were conducted along the northeast coast of the USA, be1ween Po rtsmouth, NH, and I he Chesapeake Bay in 1988 and 1990, to determine the population distribulion of Aureococcus anoplwgefferens, lhe chrysophyte responsible for massive and destructive 'brown tides' in Long Island and Narraganselt Bay beginning in 1985. A species-specific immunofluorescent technique was used to screen water samples, with positive identification possible at cell concentrations as low as 1020 cells ml • Bolh years, A.anophagefferens was detected at numerous stations in and around Long Island and Barnegat Bay, J , typically at high cell concentrations. To the north and south of this 'center·, nearly half of the remaining stations were positive for A.anophagefferens, but the cells were always at very low cell concentrations. Many of the positive identifications in areas distant from Long Island were in waters with no known history of harmful brown tides. The species was present in both open coastal and estuarine locations, in salinities between 18 and 32 prac1ical salinity units (PSU). The observed population distributions apparently still reflect the massive 1985 outbreak when this species first bloomed, given the number of positive locations and high abundance of A.anophagefferens in the immediate vicinity of Long Island. However, the frequent occurrence of I his species in waters far from this populalion 'center· is disturbing. Aureococcus anophagefferens is more widely distributed than was previously thought. Numerous areas lhus have the potential for destructive brown lides such as those associated with the sudden appearance of lhe species in 1985.