The effects of nutrient enrichment on algae and macroinvertebrates in the everglades: A review

Abstract

Over 400 metric tons of phosphorus and 12,000 metric tons of nitrogen flow annually into the northern part of the Everglades. Data describing the effects of nutrient enrichment, especially phosphorus, on algae and macroinvertebrates in the Everglades are reviewed and compared to our preliminary, results. The Everglades is a harsh aquatic environment characterized by high summer temperatures (>35 C) and fluctuating, low oxygen conditions. In unenriched habitats, oxygen concentrations at the algae-water interface can vary from 200% supersaturation (approximately 30 mg/l) during the day to less than 3.0 mg/l at night. Although surface water carbon dioxide concentrations from nighttime respiration are depleted by noon of the following day, pH remains circumneutral because of high bicarbonate concentrations. Growths of cyanobacteria, often exceeding 6 cm in thickness, characterize unenriched open water habitats. Despite longstanding suggestions to the contrary, there is no evidence that the cyanobacterial mat is consumed by higher trophic levels. Annual net daily metabolism in unenriched habitats ranges from -0.065 to 2.01 g C m -2 d ' with an average of 0.92 g C m--' d '. Previous reports concerning the effects of P enrichment on oxygen and macroinvertebrates are confounded by comparisons between habitats that differ with respect to hydroperiod, water depth, and macrophyte species composition in addition to nutrient loading. Nutrient enrichment causes disintegration of the abundant cyanobacterial mat and a shift in algal species composition and relative abundance. Annual primary production and algal biomass were 2.9 and 3.0 x greater, respectively, in phosphorus enriched conditions compared to unenriched areas. Algal diversity, however, was not significantly different in enriched compared to unenriched habitats. Our preliminary analyses indicate that macroinvertebrate diversity and density are higher within enriched compared to unenriched areas if similar habitat types (submerged macrophyte beds) are sampled. Also, there is no evidence that "fish kills" caused by anaerobiosis occur with greater frequency in enriched compared to unenriched areas.

Response of macroinvertebrates and small fish to nutrient enrichment in the northern Everglades

“…Depletion of oxygen caused by a high biological oxygen demand is the most important mechanism causing a decline in species diversity within most enriched lakes and streams (Wetzel 1983, Mason 1991. However, oxygen concentrations in the Everglades within unenriched sloughs fluctuate on a diel basis between 0.0 and 4.5 mg/Ljust before sunrise and from 25.0 to 30.0 mg/L by mid-day (Rader andRichardson 1992, SFWMD 1992). Everglades invertebrates (Rader 1994) and small fish (Kushlan 1974, Kushlan 1979) are adapted to naturally low and fluctuating levels of oxygen that characterize shallow, stagnant bodies of water.…”

“…Except for a single taxon (Palaemonetes paludosus), there was no evidence that nutrient-enriched water from agricultural runoff has had a direct harmful effect on the foodweb. Not only was the abundance of food (macroinvertebrates and small fish) higher within the enriched area, but the enriched area represents less than 2% of the total Everglades habitat available for colonization and nesting of wading bird populations (Rader and Richardson 1992). The decline of wading bird populations is probably not associated with nutrient enrichment in the northern Everglades.…”

“…Species that cannot tolerate fluctuating oxygen conditions and some degree ofanaerobiosis have probably never been able to successfully colonize the Everglades. Therefore, if enrichment does cause a decline in the diel fluctuation of oxygen, an unproven assumption (Rader and Richardson 1992), it probably would have no or little impact on the invertebrate assemblage which is already adapted (see Rader 1994) to fluctuating oxygen concentrations.…”

“…Environmental characteristics of each site are given in Table 1. We compared open water/slough habitats containing submerged macrophytes and abundant growths of algae, since they are the centers of biological diversity in the Everglades (Gleason 1974, Rader andRichardson 1992).…”

Response of everglades plant communities to nitrogen and phosphorus additions

“…and emergent macrophytes (Eleocharis spp., Rhynchospora spp., Panicum spp. ), and they are generally flooded throughout the year (Loveless 1959, Rader andRichardson 1992).…”