Grazing and Windows of Opportunity for Dinoflagellate Blooms

Dinoflagellates can cause harmful algal blooms. The fact that high densities of some dinoflagellates appear to be resistant to grazing is probably the reason that grazing pressure has not often been considered in prediction of dinoflagellate blooms. However, in early stages of bloom formation, grazing may prevent blooms. The research objectives of this proposal are to determine if "windows of opportunity" occur when and where grazing pressure is low on dinoflagellates; if "windows" are a necessary condition for initiation of blooms; and, to define the physical and biological parameters that can create them. The following hypotheses will be tested:

1. Microzooplankton community grazing is greater than mesozooplankton community grazing on small (<25mm) dinoflagellates.
   
2. Microzooplankton community grazing coefficients (g) are usually higher than growth rates (u) of small dinoflagellates, and prevent net growth.
   
3. Blooms occur when, in addition to environmental conditions being conducive to growth, there is a "window" of reduced grazing pressure in which m>g. There is no bloom if g>u.
   
4. "Windows" occur following the spring diatom bloom because microzooplankton that consume dinoflagellates are low in abundance, due to lack of appropriate sized food and top down control by copepods.
   
5. "Windows" occur after influxes of freshwater that alter boundaries between oligohaline and mesohaline waters. Transition areas that have sufficient nutrients and light, but where densities of microzooplankton grazers are low, support blooms.

The hypotheses will be tested with in situ observations, on-deck experiments and carboy experiments in two sub-estuaries of the Chesapeake Bay, the Choptank and Patuxent Rivers, where blooms of potentially harmful <25mm dinoflagellates, including Prorocentrum minimum and Gyrodinium galatheanum, are common. Although the biomass of the spring diatom bloom and summer production in Chesapeake Bay are predictable from the magnitude of the spring freshet, and hence input of nutrients, dinoflagellate blooms are not predictable. Understanding the role of grazing in preventing initiation of bloom should lead us to a better understanding of why blooms occur in the Chesapeake Bay and other coastal waters. If grazing is an important factor then it is particularly important to ask how input of nutrients, input of toxic contaminants, over-fishing and manipulation of river flows is altering food web structure and hence grazing pressure on dinoflagellates.