The brown tide is a marine microalgal bloom which has appeared in Long Island's Peconic and South Shore estuaries as well as in Narragansett Bay (Rhode Island), Barnegat Bay (New Jersey), Delaware's inland bays, Maryland's coastal bays, and in South Africa. It is caused by a particularly small (2-3 microns in diameter) and previously unknown phytoplankton species called Aureococcus anophagefferens (meaning "golden sphere that causes cessation of feeding").
The brown tide was first detected in Suffolk County waters in June of 1985. Researchers at the University of Rhode Island's Graduate School of Oceanography, using electron microscopy, placed the causative organism in a new genus and species, Aureococcus anophagefferens. The Aureococcus bloom, locally referred to as the "brown tide", has persisted in various Suffolk County embayments (particularly Flanders Bay, West Neck Bay, Quantuck Bay, and in Great South Bay) although unpredictably and with variable intensity.
What are some impacts of the brown tide?
When a brown tide bloom exceeds a concentration of approximately 250,000 cells/ml, affected waters typically turn a murky brown and water transparency decreases to less than 2 feet. This has an immediate aesthetic impact on residents and tourists, in addition to having a serious impact on eelgrass (Zostera marina) beds due to reduced light penetration. Eelgrass is actually a flowering plant that requires light to manufacture its food (through photosynthesis). Because they provide spawning and nursery grounds for fish and shellfish and play a critical role in estuarine food webs, eelgrass beds are an extremely important component of marine ecosystems.
Perhaps one of the most severe impacts of the brown tide has been on the once prosperous Peconic Bay scallop industry. This resource, once estimated to be worth more than $2 million dollars annually, was virtually eradicated by the brown tide. Attempts to revive the industry through seeding operations have been marginally successful. Although the last major brown tide bloom was in 1995 (in the Peconic estuary), the bay scallop harvest has still not recovered to levels that existed prior to the 1985 bloom.
Other shellfish species, including hard clams, oysters, and mussels have also been seriously affected. Declines in finfish landings are an additional impact, as fish seem to avoid bloom areas.
Does the brown tide pose a public health threat?
Although the brown tide has had a serious effect on natural resources, the local economy, and the general aesthetic value of the estuaries, it is not known to be harmful to humans. Although reduced water transparency may present certain safety risks to bathers, there is no evidence that either consuming finfish or shellfish from affected waters, ingesting the brown tide, or bathing in brown tide containing waters, has any harmful effects.
What causes brown tide blooms?
Although advances have been made regarding the identification and characterization of the brown tide organism and its growth needs, the causes of brown tide are still not clear. The input of conventional inorganic nitrogen and phosphorus nutrients (ammonia, nitrate, and phosphate) apparently do not trigger the onset of the blooms, although the availability of organic nitrogen compounds may play a role. In the Peconic Estuary, groundwater inputs affecting the relative amounts of dissolved inorganic and organic nitrogen may be an important factor in the onset of brown tide blooms.
Other factors identified as having potential involvement in the growth and predominance of brown tide include the following:
- chelators such as citric acid and trace metals such as iron, selenium, vanadate, arsenate, and boron
- the failure of potential grazers (microzooplankton) to keep brown tide in check
- meteorological and climatological factors - it has been postulated that reduced flushing due to a change in wind induced subtidal sea level oscillation, results in the retention of land-derived nutrients that may stimulate brown tide blooms
- physio-chemical limits - monitoring data collected by the Suffolk County Dep't. of Health Services suggest that salinities in excess of 26 parts per thousand and temperatures between 20-25 degrees C may be factors associated with the occurrence of major blooms
- the presence of filter feeding clams that may play a role in limiting bloom development
- in low light conditions (typical of the turbid Great South Bay) the organism can supplement photosynthesis with the uptake of organic compounds, giving it a competitive advantage over other phytoplankton species