Extracted from:

Donald F. Boesch, Donald A. Anderson, Rita A. Horner, Sandra E. Shumway,Patricia A. Tester and Terry E. Whitledge. 1997. Harmful Algal Blooms in Coastal Waters: Options for Prevention, Control and Mitigation. NOAA Coastal Ocean Program, Decision Analysis Series No. 10, Special Joint Report with the National Fish and Wildlife Foundation, February 1997.

Site developer's note: genus and species names of algae are indicated in red, rather than standard underlining (not a webpage option unless a link...) or italics, in order to make the names easier to find and read.

Brown Tide Blooms

In 1985, a massive bloom of the very small microalga Aureococcus anophagefferens occurred in the coastal bays of Long Island, New York. Concurrent with the blooms around Long Island, blooms were recorded in Narragansett Bay (Rhode Island) and Barnegat Bay (New Jersey) (Sieburth et al. 1988, Olsen 1989). The blooms reached very high densities and were commonly referred to as "brown tide" due to the striking discoloration of the water (Cosper et al. 1990). A similar event occurred in Texas a short time after an extremely cold, windy even in December 1989. Subfreezing temperatures coincident with low tides killed millions of finfish and benthic organisms in Laguna Madre (Buskey and Stockwell 1993). The decomposition of the dead biomass produced and order of magnitude increase of inorganic nitrogen nutrients relative to normal levels (Whitledge 1993). Consequently, high densities of the species being described as new to science as Aureoumbra lagunesis developed resulting in the formation of a "brown tide," which enigmatically has persisted until this day, seven years later.

The blooms in Texas and Long Island had substantial ecological and economic effects. In both regions the dense algal blooms resulted in decreased light penetration and reductions in the extent of seagrass beds, especially in Texas where 20 percent coverage has been lost in water depths below one meter (Onuf 1996). In Long Island waters, brown tide blooms (BTBs) had a severe impact on commercially valuable bay scallops, affecting bays which contribute more than 80% of New York State's bay scallop harvest (Cosper et al. 1987). This fishery is worth an estimated $2 million per year. IN addition other shellfish, including the commercially valuable hard clam, have also been affected.

The brown tide has recurred in Long Island embayments several years since 1985 and various theories exist regarding its formation, such as increased freshwater flow drought, low organic nitrogen concentrations, high iron availability, capacity of Aureococcus for growth on organic nitrogen sources, decreased exchange with ocean water, and variations in rainfall and groundwater inputs (Nixon et al. 1994; Cosper et al. 1990). In contrast to the Long Island BTB which has an annual cycle of retreat and redevelopment, Aureoumbra has bloomed continually since 1989 in Texas embayments and lagoons with a limited water circulation and exchange. Restricted circulation promotes nutrient and biomass accumulation by retaining dissolved and particulate materials in the ecosystem, thereby maintaining availability of vital elements. In the presence of these regenerated nutrients, the growth rate of the BTB organism exceeds advective losses and bloom development occurs quickly and persists for long periods of time.

However, the causes of an initiation of a BTB may be different than the factors responsible for the increased prevalence of blooms. The existence of the bloom nay even enhance some processes for self-perpetuation. One example of this is continual regeneration of nutrients from existing biomass. Nevertheless, the persistence of a bloom in the longer term also requires additional influx of nutrients from terrestrial or atmospheric sources. Physical mixing of the water column and resuspension of sediments are factors that must be considered as potential mechanisms of bloom persistence. There is some evidence that there may be benthic phase or a mixing related resuspension of cell aggregates that enhances or maintains a BTB (Stockwell et al. 1993). Aside from nutrient regeneration and mixing processes, one of the prime "maintenance" mechanisms is lack of grazing by water column and benthic populations (Buskey and Stockwell 1993). These processes control levels of phytoplankton biomass under normal conditions. Both Long Island and Texas brown tide organisms were found to retard zooplankton grazing in field and laboratory samples through an apparent release of inhibiting substances (Buskey and Hyatt 1995). Interestingly, microzooplankton grazing was reduced only when chlorophyll concentrations exceeded 10 (mu)g/l (Buskey et al. 1996). Based on discussions during the Port Aransas meeting, other factors which could govern the persistence of BTB include decreased competition from the natural phytoplankton assemblage and lower-than-normal loss rates sinking (e.g., via intensifying mixing and resuspension) and viral infections.

Other than reductions in the extent of seagrass beds due to reduced light levels, Texas brown tides have not been linked with obvious reductions of other living resources. Whether due to population rebound following the 1989 freeze-related mortality, the effects of reduced fishing pressure associated with conservation-related regulations, or the contributions of restocking efforts, all or any of these factors have allowed populations of highly prized game fish, such as red drum and spotted sea trout, to increase to higher levels than before the BTB (Mceachong and Fuls 1996). How these species are fished has, however, also changed. As a result of the heightened turbidity of previously clear waters, lures that depend on visibility are not as effective, making fishing less attractive to sport fishers. There is some evidence that larval fish populations are reduced in areas experiencing a BTB and survival is only 15-20% of controls in laboratory and fish hatchery exposures to BTBs. Whether this is of any consequence to the stocks or whether there are delayed effects which may result in a future decline in adult populations are unknown at this point.

UNIVERSITY OF MARYLAND College of Ag & Natural Resources Department of Veterinary Medicine Aquatic Pathobiology Center