Paul V. McCormick

A proposed framework for developing indicators of ecosystem health

Considerations involved in developing a suite of indicators to monitor regional environmental health, similar in conception to management use of ‘leading economic indicators’, are described. Linkages between human activities and well being and the state of the environment are considered essential to the evaluation of general environmental health. Biogeochemical and socioeconomic indicators are mutually affected by environmental degradation and examples of both categories of indicators are described. Desirable properties in indicators of environmental health vary with their specific management use. Different indicators are called for when collecting data to assess the adequacy of the environment, monitor trends over time, provide early warning of environmental degradation, or diagnose the cause of an existing problem. Tradeoffs between desirable characteristics, costs, and quality of information are inevitable when choosing indicators for management use. Decisions about what information to collect for which purpose can be made more rationally when available indicators are characterized and matched to management goals.

Quantifying Periphyton Responses to Phosphorus in the Florida Everglades: A Synoptic-Experimental Approach

Water quality and periphyton were sampled along a nutrient-enrichment gradient in the northern Everglades to develop correlative relationships between periphyton taxonomic composition and changes in water chemistry. A controlled phosphorus-enrichment experiment was conducted concurrently to develop causal relationships between periphyton changes and phosphorus enrichment. Periphyton changes along the gradient were most strongly related to changes in total phosphorus (TP), but also were correlated with other potentially limiting nutrients (e.g., nitrogen, iron). Interior marsh stations (water column TP = 5-7 μg/L) were dominated by calcareous periphyton mats composed of the cyanobacteria Scytonema hofmanii and Shizothrix calcicola (27-70% of total biovolume) and an abundance of diatom epiphytes (11-49% of total biovolume). This assemblage was replaced at stations having slightly elevated phosphorus concentrations (TP ≥ 10 μg/L) with a periphyton assemblage dominated by filamentous green algae including Spirogyra and Mougeotia (80-99% of total biovolume) and containing few diatom epiphytes (<6% of total biovolume). These same taxonomic shifts occurred in response to experimental enrichment (0-12.8 g P m-2 yr-1) of interior marsh plots with phosphorus, thus confirming that these periphyton changes were due to phosphorus enrichment by human activities. Periphyton assemblages at marsh stations where phosphorus concentrations were highest (TP = 42-134 μg/L) were not reproduced experimentally, suggesting that environmental factors other than phosphorus may determine periphyton taxonomic composition in highly enriched areas of the marsh. The periphyton assemblage is a critical component of the Everglades ecosystem and changes documented here in response to phosphorus enrichment constitute a significant ecological impact on this ecosystem.

Periphyton-Water Quality Relationships along a Nutrient Gradient in the Northern Florida Everglades

We monitored a 14-km nutrient gradient in the northern Everglades to identify statistical relationships between periphyton and water-quality changes caused by canal discharges into the marsh. Water chemistry measurements were taken at 15 sampling stations every 2 wk over a 20-mo period to quantify changes in major ions along the gradient. Standard algal bioassays were conducted using water from each station to identify trends in nutrient limitation and algal growth potential along the gradient. Patterns of periphyton biomass accumulation and taxonomic composition on artificial substrata were determined in situ during 6 sampling events across seasons. Concentrations of most ions decreased by <25% along the gradient, whereas average total phosphorus (TP) decreased from 150 μg/L at peripheral marsh stations (near canal inflows) to ≤10 μg/L at interior stations (>8 km from inflows). Limiting nutrient assays showed a shift from P limitation at interior stations to possible limitation by other nutrients at peripheral stations. Both algal growth potential and biomass accumulation decreased with increasing distance from the canal, and stepwise regression analysis showed that these changes were best explained by changes in TP along the water-quality gradient. Changes in periphyton taxonomic composition, analyzed using principal components analysis, were also related strongly to distance from canal discharges and to TP. In particular, diatom species indicative of low TP (e.g., Anomoeoneis vitrea, Mastogloia smithii) were consistently replaced by eutrophic indicator species (e.g., Gomphonema parvulum, Nitzschia amphibia) at TP concentrations between 10 and 20 μg/L. The Everglades periphyton assemblage is sensitive to phosphorus enrichment and may provide one of the first reliable indications of eutrophication in the marsh.

Effects of Snail Grazing on Benthic Algal Community Structure in Different Nutrient Environments

The effects of interaction between nutrient availability and grazing intensity on the structure of benthic algai assemblages were investigated in an ephemeral stream. Small (250 ml) enclosures containing substrata releasing either: 1) no nutrient, 2) 0.5 M KH2 PO4 or 3) 0.5 M NaNO3 were exposed for 19 d to one of five densities of the snail Goniobasis ranging from 0 to 160 snails/m2. Algal standing crop (total cell density and community biovolume) on coverslips inside each enclosure decreased with increased grazing pressure in control and nitrate-enriched environments, but increased at low grazing pressure in the phosphate-enriched environment. Responses among constituent populations differed for three reasons: 1) stalked and loosely attached diatoms were more susceptible to snail grazing than firmly attached, prostrate algae; 2) some species were stimulated by phosphorus enrichment and/or grazing while others were not; 3) algal species differed in their response to interactions between grazing and nutrient enrichment. Two alternative hypotheses may explain this interaction: 1) grazers stimulate the growth of understory species by removing overlying cells; 2) excretion of nitrogenous compounds by grazers enhances the effects of phosphorus enrichment alone. Although diversity peaked at intermediate grazer densities in unenriched and nitrate-enriched environments, no significant change was found in phosphate-enriched environments, where the community was dominated either by grazer-sensitive populations that increased faster than they were grazed, or by grazer-resistant algae.

MECHANISMS OF BENTHIC ALGAL SUCCESSION IN LOTIC ENVIRONMENTS

Although seasonal patterns of algal succession have received considerable attention, few studies have attempted to elucidate the processes that contribute to short- term algal succession following disturbance. We framed the present study around four general mechanisms of succession in order to investigate the contribution of species-specific autecologies and interspecific interactions to observed short-term patterns of algal succes- sion in a productive third-order stream in Kentucky, USA. Observations in different current environments in the stream during two consecutive winters were used to classify dominant species as early or late successional based on changes in relative abundance through time on newly exposed substrates. Assemblages of the same age but differing in the relative abundance of early- and late-successional species were developed in streamside channels recreating both current environments to measure growth parameters (e.g., reproduction) of dominant species and to test for interspecific interactions. Of the five dominant species of algae in the winter assemblage, three species were consistently classified as early successional (i.e., decreased in relative abundance with in- creasing assemblage age) and two species as late successional. Early-successional species differed in their growth form and strategy for initially dominating the substrate, having high densities in the water column, high probabilities of attaching, or fast early reproductive rates. Late-successional species exhibited a more extended growth form and had the highest per capita reproductive rates during later growth. Late-successional species reduced the reproductive rate of early species as succession proceeded in both current environments but were not themselves inhibited by biovolume increases. While differences in growth strategies between early- and late-successional species indicated the importance of passive tolerance mechanisms of succession, density-dependent interactions during community development were consistent with active tolerance mechanisms. As in terrestrial plant communities, successional patterns in benthic algal assemblages appear to result from several processes that defy explanation by a single mechanistic model.

Grazer control of nutrient availability in the periphyton

SummaryBenthic algal assemblages are regulated by both abiotic (e.g., nutrient) and biotic (e.g., grazing) constraint. The objective of this study was to determine how changes in these two factors affected the structure of an algal assemblage in an ephemeral stream. Coverslips were incubated for 21 days in enclosures containing one of three nutrient environments (ambient, phosphorus-enriched, or phosphorus and nitrogen enriched) and one of four densities of the snail Gonibasis (0, 40, 80, or 120 snails/m2) and examined directly to enumerate the algal assemblage. The effect of grazing on algal biomass was dependent on the nutrient environment. An overstory of diatoms was susceptible to removal by grazing and was not strongly affected by nutrient enrichment. An understory of Stigeoclonium was more resistant to grazing and responded strongly to nutrient enrichment only in the presence of grazers. Snail grazers may mediate nutrient availability to the understory indirectly by removing overlying cells or by direct excretion of nutrients. Multiple interactions occur between benthic herbivores and algae, and, as shown here, some of them are positive and involve modifications of the nutrient environment.

Lotic Protistan Herbivore Selectivity and Its Potential Impact on Benthic Algal Assemblages

Despite the ubiquitous presence of microscopic herbivores such as protozoa in aquatic ecosystems, research on benthic herbivory has largely focused on the role of macroscopic grazers. The present study shows the potential for protozoan herbivores to affect the structure of benthic algal assemblages through selective grazing. The abundance and feeding preferences of three common algivorous protozoans, the ciliates Chilodonella sp. and Trithigmostoma cucullulus and the sarcodine Pelomyxa sp., were determined in a productive, third-order stream and in stream-side channels which contained different densities of two macrograzers and algal assemblages with different physiognomies and species composition. Although the species composition of the ingested algal contents of individual micrograzer cells varied, all three micrograzer populations exhibited selectivity in their feeding on dominant diatom species. Selectivity varied among microherbivore species and with the abundance of indigenous macrograzers. Although the smallest of the three microherbivores (Chilodonella) appeared to be limited to grazing smaller diatom species, selectivity by the two larger species was not strongly related to size or growth form of diatom prey. Selective feeding was still prevalent when the three microherbivore species were considered together as a guild, indicating the potential for an impact on diatom assemblage structure. Although initial predictions of grazing rates of herbivorous protozoa on benthic diatoms suggest that such impacts can be substantial, more detailed studies of protozoan production potentials and microherbivore-algal interactions will be required to confirm or reject these preliminary estimates.

A simple, cost-effective multispecies toxicity test using organisms with a cosmopolitan distribution

Difficulties in making accurate, ecosystem-level predictions of environmental effects of chemicals, mixtures, and effluents based solely on the results of tests on single species have necessitated the development of more environmentally realistic, predictive testing methods. This paper describes a multispecies, community-level toxicity test based on the colonization of artificial substrates by microbial species. Tests examined the colonization of initially barren polyurethane foam artificial substrates by Protozoa from a species source colonized in a natural system. Differences in colonization were examined in microecosystems amended with low levels of cadmium, a very toxic heavy metal, and TFM, an organic biocide used against larval sea lamprey. Tests examined differences in colonization over 28 days. For cadmium, effect levels were estimated to be near 1 μg 1−1, in the low range of effect levels determined from chronic single species tests. For TFM, effect levels were estimated to be between 1 and 10 ppm, overlapping the concentrations used in environmental applications. The colonization response, which depends on naked microbial cells reproducing and migrating through toxicant amended water to new substrates, is very sensitive. Tests based on colonization can be adapted to use species from a target receiving system or can use species from a designated natural source. Field validation of these tests can employ nearly identical methods to those used in laboratory studies to assess the accuracy of predictions based on test system data.

Estimating ecotoxicological risk and impact using indigenous aquatic microbial communities

Emphasis has increased on accuracy in predicting the effect that anthropogenic stress has on natural ecosystems. Although toxicity tests low in environmental realism, such as standardized single species procedures, have been useful in providing a certain degree of protection to human health and the environment, the accuracy of such tests for predicting the effects of anthropogenic activities on complex ecosystems is questionable. The use of indigenous communities of microorganisms to assess the hazard of toxicants in aquatic ecosystems has many advantages. Theoretical and practical aspects of microbial community tests are discussed, particularly in related to widely cited problems in the use of multispecies test systems for predicting hazard. Further standardization of testing protocols using microbial colonization dynamics is advocated on the basis of previous studies, which have shown these parameters to be useful in assessing risk and impact of hazardous substances in aquatic ecosystems.

Response of protistan assemblages to a model toxicant, the surfactant C12-TMAC (dodecyl trimethyl ammonium chloride), in laboratory streams

Abstract Aquatic microbial communities contain many species performing several critical functions in aquatic ecosystems and, thus, may provide robust indicators of stress. Effects of the cationic surfactant dodecyl trimethyl ammonium chloride (C12-TMAC) on benthic microbial communities in experimental stream channels were assessed. Tests were performed with microbial communities: (1) developed in the stream channels for 14 days prior to exposure; (2) developed in the presence of the chemical; (3) developed in the relatively unimpacted headwaters of the stream providing water to the facility for 16 days prior to exposure. Gross indicators of community structure, such as the taxon richness and composition of the protozoan and diatom assemblages tended to be unaffected by the chemical. Effects on growth processes and trophic relationships were generally transitory and occurred at moderate concentrations (250 μg/l) of cationic surfactant. Most sensitive single species responses to this chemical in previous laboratory tests were more sensitive than overall community responses examined here and elsewhere. However, broad assessments of risk using these communities allow for: (1) validation of single species responses, and (2) predictions of potential long-term effects on natural ecosystems unavailable from single species tests. This should allow for increased precision in risk assessments.