| | | 7.4 Eutrophication and Phytoplankton Species Selection and Responses |
The relationships between eutrophication and phytoplankton biomass are more evident than the effects of anthropogenic nutrient enrichment on individual phytoplankton species. Field studies have detected changes in species composition and abundance in a variety of nutrient-enriched environments, including the Dutch Wadden Sea [65][393], the Baltic Sea [258][354], the Black Sea [47][48] and New York Bay [304]. As already pointed out, the increase in nutrients during the initial phase of eutrophication leads to higher phytoplankton biomass through yield-dose kinetics. During these initial stages, particularly during the winter-spring period, the indigenous flora (most often diatoms) usually increases in abundance without novel changes in species behaviour occurring. The Si-requiring diatoms are generally considered to be the `most desirable' bloom species with regard to grazer suitability, trophic value and water quality. Should nutrification continue to increase, a change in the species composition and the size structure of the phytoplankton community may result and potentially affect energy flow in the impacted ecosystem [136]. In extreme cases, the impacts of altered phytoplankton species composition, abundance, size structure and bloom events in response to nutrient enrichment will cascade through upper trophic levels and reform food web structure [472]. This latter (secondary) effect will tend to blur the primary responses (i.e. phytoplankton behavior) to eutrophication and mask cause and effect. The nutrient-induced reformation of ecosystems is an extremely complex and poorly understood ecological process.
The shift in phytoplankton species that has attracted the most interest is the shift in abundance (blooms) from diatoms to other non-motile species and flagellates -- a functional group shift. Functional groups and their shifts are of interest because of significant differences in their physiology and ecological impacts. There is special interest in the diatom:flagellate ratio as a potential indicator of eutrophication since the global increase in harmful microalgal blooms (HABs) is primarily a flagellate species phenomenon [see also Chapter , this volume]. It has been hypothesized that the diatom:flagellate ratio should decrease with increasing nutrient enrichment, and consequently might serve as an indicator of eutrophication status. There is some supporting evidence for this from Kastela Bay, Croatia, where a progressive, long-term increase in anthropogenic nutrient has been accompanied by a 10-fold decrease in the ratio of diatom to flagellate abundance [307]. The primary nutrient expected to regulate the shift in functional groups from diatoms to flagellates is silica, which is required by diatoms but not by other microalgal groups exclusive of silicoflagellates [348][431]. Silica concentrations and ratios with N and P are altered by eutrophication, with the degree and pattern of change influenced by the chemical nature of the waste water being discharged [348][103]. Silica is assimilated by diatoms stoichiometrically in the Redfield Ratio (Eq. 1) in atomic proportions of 1:1 with N, and 16:1 with P. At Si:N supply ratios of <1:1, diatoms will be Si-limited, and N-limited at Si:N supply ratios >1:1. Smayda [431], based on an evaluation of long-term blooms and nutrient conditions in various regions, has suggested that anthropogenic enrichment of N and P has led to long-term declines in the ratios of Si:N and Si:P which potentially favor non-diatom blooms in such impacted regions. Mescocosm experiments led Egge et al. [144] to suggest that there is a threshold of approximately 2 µM Si, below which "diatoms, as a group, are outcompeted by the `flagellate group'". The merit of the Si ratio and threshold concepts as eutrophication switches that result in species shifts and altered community abundance is still under investigation. But it is clear that the species-specific responses to these proposed Si effects are under multifactorial control rather than are simple linear responses. For example, Sommer's [435] experiments showed that diatoms became dominant at Si:N ratios >25:1, while flagellates were superior competitors at lower ratios. Although irradiance did not significantly influence the competition between diatoms and flagellates in these experiments, it was important in the competition among diatom species.
There has been much greater interest in N:P ratios since N and P have been the primary nutrients focused on by phytoplankton ecologists. Niemi [] was among the first to invoke N:P ratio control of cyanobacterial blooms of Nodularia and Aphanizomenon species in the Baltic Sea. In his view, these N-fixing species were able to capitalize on the elevated phosphorus levels occurring then, leading to their competitive advantage over other functional groups. Similar N:P regulation of Nodularia spumigena in a P-enriched Australian estuary has been reported [294]. In Tolo Harbour, Hong Kong, a long-term increase in harmful algal and red tide blooms has accompanied eutrophication associated with a marked increase in the human population (see [431]). Hodgkiss and Ho [233] report that the annually averaged N:P ratio decreased from 20:1 to 11:1 over a seven year period, during which the number of dinoflagellate blooms increased. In the Dutch Wadden Sea, a long-term increase in abundance of Phaeocystis pouchetii and its dominance of the annual phytoplankton cycle has occurred in response to nutrient enrichment via riverine discharge [277]. Altered nutrient ratios appear to have played a important role in this exploitation. Riegman et al. [393] have shown that the average annual dominance of Phaeocystis was inversely related to the average N:P ratio during its growth season (April - September), and that the ratio of NH4 : NO3 influenced life cycle stage. In laboratory experiments, Riegman [392] found that Phaeocystis in competition against other species became dominant at N:P molar ratios of <= 7.5 and approached monospecific bloom formation at N:P ratios of 1.5. The successful competition of Phaeocystis against diatoms in achieving Wadden Sea preeminence was also linked to lower Si concentrations [144]. The catastrophic bloom of Chrysochromulina polylepis in the Kattegat and Skagerrak during 1988 has been linked to N:P ratio effects on bloom magnitude and toxicity [301], although the role of nutrients in this bloom remains controversial. Additional information on the effects of nutrient ratios on phytoplankton species selection will be found in Anderson et al. [9] and Granéli (HAB Chapter, this volume).
| | | 7.4 Eutrophication and Phytoplankton Species Selection and Responses |