One of the explanations given for the increased incidence of HAB outbreaks worldwide is that these events are a reflection of increased pollution and nutrient loading in coastal waters. Some argue that we are witnessing a fundamental change in the phytoplankton species composition of coastal marine ecosystems throughout the world due to the changes in nutrient supply ratios from human activities [ Smayda, 1990]. There is no doubt that this is true in certain areas of the world where pollution has increased dramatically. It is perhaps real, but less evident in areas where coastal pollution is more gradual and unobtrusive. A frequently cited dataset from an area where pollution is a significant factor is from Tolo Harbor in Hong Kong, where population growth within the watershed grew 6-fold between 1976 and 1986. During that time, the number of observed red tide events increased 8-fold [ Lam and Ho, 1989]. The underlying mechanism is presumed to be increased nutrient loading from pollution that accompanied human population growth. A similar pattern emerged from a long-term study of the Inland Sea of Japan, where visible red tides increased steadily from 44 per year in 1965 to over 300 a decade later, matching the pattern of increased nutrient loading from pollution [ Murakawa, 1987]. Japanese authorities instituted effluent controls in the mid-1970's, resulting in a 50% reduction in the number of red tides that has persisted to this day.
These two examples have been criticized, since both could be biased by changes in the numbers of observers through time, and both are tabulations of water discolorations from algal blooms, not just toxic or harmful episodes. Nevertheless, the data demonstrate that coastal waters receiving industrial, agricultural, and domestic effluents, which frequently are high in plant nutrients, do in fact experience a general increase in algal growth. These nutrients can stimulate or enhance the impact of toxic or harmful species in several ways. At the simplest level, toxic phytoplankton may increase in abundance due to nutrient enrichment but remain as the same relative fraction of the total phytoplankton biomass (i.e. all phytoplankton species are affected equally by the enrichment). Alternatively, some contend that there has been a selective stimulation of HAB species by pollution. This view is based on the nutrient ratio hypothesis [ Smayda, 1990] which argues that environmental selection of phytoplankton species is associated with the relative availability of specific nutrients in coastal waters, and that human activities have altered these nutrient supply ratios in ways that favor harmful forms. For example, diatoms, the vast majority of which are harmless, require silicon in their cell walls, whereas most other phytoplankton do not. Since silicon is not abundant in sewage effluent but nitrogen and phosphorus are, the N:Si or P:Si ratios in coastal waters have increased through time over the last several decades. Diatom growth in these waters will cease when silicon supplies are depleted, but other phytoplankton classes (which have more toxic species) can continue to proliferate using the ``excess'' nitrogen and phosphorus.
This concept is controversial, but is not without supporting data. A 23-year time series off the German coast documents the general enrichment of coastal waters with nitrogen and phosphorus, as well as a four-fold increase in the N:Si and P:Si ratios [ Radach et al., 1990]. This was accompanied by a striking change in the composition of the phytoplankton community, as diatoms decreased and flagellates increased more than ten-fold.
As coastal communities and countries struggle with pollution and eutrophication issues, the implications of these studies are profound. Increasingly, the possible stimulation of HAB species by domestic or industrial effluent is being raised by those in opposition to public works projects. One example is a proposed new sewage outfall that will release up to 1 billion gallons of effluent each day into Massachusetts Bay at a point near the pathway of the coastal current described above for the PSP dinoflagellate A. tamarense. Opponents of the project cite the time series described above and argue that an adverse impact of the outfall will be an increase in harmful or toxic algal species within the Bay. The stakes in this controversy are huge and the scientific uncertainty significant. The public, the press, and regulatory officials are demanding predictions and answers, yet their expectations exceed our present capabilities. Competitive outcomes in phytoplankton species selection and succession cannot yet be predicted, nor can the relative effects of natural versus anthropogenic factors be resolved. To address the concern that the phytoplankton species composition will change with the different quantities and ratios of nutrients in the effluent from the new outfall, ecosystem-level models of the Bay are required that are a decade or more away. Even when the focus is narrowed to a few key HAB species, their responses within the Bay ecosystem cannot be estimated with any accuracy because their nutrient requirements have not been well-characterized in laboratory studies, nor do we even have ways to determine their nutrient status during present-day blooms.
The potential stimulatory influence of anthropogenic nutrient inputs on HAB incidence is certainly one of the more pressing unknowns we face, and it will require a focused commitment of resources and effort greatly in excess of what has been devoted to the topic until now. Time-series analysis of existing data bases for phytoplankton communities and variables such as major nutrients or pollutants is required, and where such data are lacking, long-term monitoring programs of at least 10-years duration must be initiated in key regions where anthropogenic changes are anticipated. Laboratory studies of the stimulatory effects of chemicals contained in effluents or terrestrial runoff are also needed, as are kinetic studies and other experiments that can quantify the nutritional requirements and uptake capabilities of HAB species.