| | | 7.5 Eutrophication, Indicator Species and Harmful Blooms |
Although marine phytoplankton respond to nutrification by increasing their biomass and through altered species behaviour, most of the results derive from uncontrolled experiments and qualitative field evidence. There is currently inadequate quantification of the eutrophication-phytoplankton response to predict the species-specific responses to acute or chronic nutrient loadings, and whether this altered behaviour will benefit or disrupt a given ecosystem. Recognition that eutrophication is a process with distinct stages in the evolution from oligotrophy to eutrophy represents a major step forward towards achieving this needed quantification. A major stumbling block lies in identifying the relevant habitat features during nutrification and the life-form requirements of the species being selected. Sellner et al. [417] based on evidence from a eutrophicated branch of the Chesapeake Bay have concluded that stratified watermasses exposed to excessive nutrient enrichment are predisposed to dinoflagellate blooms. However, Smayda & Reynolds [429] recognize nine different combinations of watermass mixing/stratification and nutrient combinations, and associated dinoflagellate life-form types having distinctive morphotype-features and habitat preferences, with three different primary strategies present. This complexity may explain why specific phytoplankton indicator species of eutrophication have yet to be found despite the long-term search for such (see [57]). Smayda & Reynolds [429] in their phytoplankton life-form concept recognize the occurrence of invasive, small- to intermediate-sized colonist species (C-strategists) which often predominate in chemically-disturbed water bodies, and whose characteristics are given in in the following (from [417]):
They suggest that habitat selection for life-form is primary, while the species to be selected and conforming to this life-form are less predictable and even stochastic. It may be that as eutrophication progresses through its various stages, changes in life-form conditions occur which determine which life-form type of phytoplankter will predominate. Perhaps it is this feature rather than the search for specific indicator species that should be pursued in future research.
In discussion of the effects of eutrophication on phytoplankton species, frequent reference has been made to harmful blooms and red tides. This partly reflects the type of information available. But a word of caution is in order with regard to the increasingly frequent and general invocation that nutrient enrichment is the cause of the global expansion and increase in harmful blooms in marine coastal waters. There is increasing reference by regulatory agencies, in work shop reports (for example, [290]), and among scientists that elevated nutrient concentrations are the major cause of the HAB epidemic and, therefore, this justifies the need to reduce nutrient loadings. While the latter is certainly desirable, there is no convincing evidence that increased nutrient loading is the general mechanism driving and accounting for most HAB events in global coastal waters [428]. There are certainly examples of an increased HAB - nutrient relationship in certain regions and nutrient driven nuisance blooms such as Phaeocystis and certain cyanobacteria [427][431], but most toxic blooms do not appear to be nutrient-stimulated (see also [9]). Long-term changes in climate, the impacts of overfishing, ballast water introductions of invasive species, and even natural variability are also contributory and intersect.
| | | 7.5 Eutrophication, Indicator Species and Harmful Blooms |