David E. Weber

Dredging impact on an urbanized Florida bayou: effects on benthos and algal-periphyton

Environmental effects of dredging events have been uncommonly reported for shallow, residential estuaries characteristic of the Gulf of Mexico region. The objective of this study was to determine the impact of hydraulic dredging on an urbanized estuary. Physicochemical quality, benthic community composition, whole sediment toxicity, periphytic algal community composition and trace metal tissue quality were determined prior to and after dredging. The effects on surface water pH, dissolved oxygen, and temperature were negligible but photosynthetically active radiation was decreased at several stations. Dredging significantly reduced benthic diversity and density (P < 0.05). However, the sediments were not acutely toxic to the epibenthic, Americamysis bahia (formerly Mysidopsis bahia); survival averaged 93% (post-dredging) and to 98% (pre-dredging). There were several post-dredging taxonomic structural changes in the diatom-dominated, periphyton community but differences in mean density and three diversity indices were not significant. Trace metal concentration in periphyton after dredging were reduced from an average of 4-65% and significantly for mercury, zinc and chromium in several areas. It was concluded that the environmental impact of small-scale dredging events in urbanized near-coastal areas, based on the selected parameters, are likely to be localized and of short-term environmental consequence. The choice of the target biota, response parameters and chemical analysis are important considerations in the environmental impact assessment of these periodic episodic events.

Effects of Substrate Salinity on Early Seedling Survival and Growth of Scirpus robustus Pursh and Spartina alterniflora Loisel

Rooted aquatic plants are being used increasingly to test the toxicity of sediments. However, effects of naturally occurring substrate constituents on most potential test species are not well understood even though their effects could affect the test results. The objective of this study was to determine the effect of substrate salinity (NaCl) on early seedling survival and growth of the emergent macrophytes, Scirpus robustus Pursh and Spartina alterniflora Loisel. Results of four 21- and 28-day toxicity tests, conducted in an artificial sediment, indicated interspecific differences in NaCl sensitivity when based on changes in shoot, root and whole plant dry-weight biomass. Concentrations of 7.8 g NaCl/ℓ and 19.2 g NaCl/ℓ first reduced early seedling biomass of S. robustus and S. alterniflora (P < 0.05), respectively, when compared to plants grown in sediment containing no measurable salinity. Seedling survival was not affected at average concentrations of 17.5 g NaCl/ℓ or less for S. robustus and 22.3 g NaCl/ℓ or less for S. alterniflora. The results indicate that substrate salinity is an important consideration in the selection of test species for laboratory phytotoxicity tests conducted with estuarine sediments, particularly if determination of chronic toxicity attributable to anthropogenic contamination is the primary objective.

Responses of wetland plants to effluents in water and sediment

Abstract Responses of two wetland vascular plants, Echinochloa crusgalli and Sesbania macrocarpa, exposed to effluents from a coke plant, a pulp mill and a wastewater treatment plant were measured in three types of tests: seed germination and early growth, seedling survival and growth in hydroponic culture, and seedling survival and growth in sand and synthetic sediments with sand, clay, silt and 3, 5, 7.5 or 10% organic content. There was no effect of effluents on germination. Growth rates were reduced significantly in all tests except for E. crusgalli exposed in sediment to effluent from a wastewater treatment plant; the effluent stimulated growth. Increasing concentrations of organic matter in sediments had little effect on toxicity of effluents.

Artificial sediments for use in tests with wetland plants

Abstract Artificial sediments are described for use in studies on rooted marsh plants. The sediments, which are similar in particle size distribution to natural sediments, are formulated from commercially available sand, silt, clay and organic matter. Average survival rates of seedlings of Echinochloa crusgalli var. crusgalli, Scirpus paludosus and Spartina alterniflora were 93.4, 90.4 and 89.4%, respectively. Average seedling weight of each species was unaffected by percentage sand, silt, clay, organic matter, pH or CEC except for S. paludosus , which required organic matter in the sediment for maximal growth and whose growth was affected by the relationship between particle size and percentage sand.

Periphyton and Sediment Bioassessment in North Florida Bay

Periphyton colonization and sediment bioassessment were used ina survey to compare the relative environmental condition ofsampling sites located in Florida Bay and four peripheral sloughareas during the summer of 1995. Periphyton biomass, pigmentcontent, tissue quality and community composition weredetermined. In addition, benthic community composition and thetoxicities of whole sediments and associated pore waters weredetermined for two species of rooted macrophytes, an epibenthicinvertebrate and bioluminescent bacteria. Several locational differences were observed for the response parameters.Periphyton biomass was significantly greater in the Taylor Riverand the least in Shell Creek (P < 0.05). Most sediments were notacutely toxic to mysid shrimp nor phytotoxic. However, sedimentsfrom the Taylor River were more phytostimulatory than others (P< 0.05). Contaminant bioresidues were similar at most sites,however, mercury, chromium and nickel concentrations weregreater for periphyton colonized in the Taylor River and TroutCreek areas. Structural characteristics of the periphytic algalcommunity usually were statistically similar but a consistenttrend of lower density and diversity was evident for ShellCreek. The benthic community was the least diverse and dense inthe Canal C-111. The results of this study provide an initialindication of differences in the role of several slough areas aspossible sources of bioavailable contaminants to Florida Baywhich warrants additional investigation.

Comparison of the effects of drilling fluid on macrobenthic invertebrates associated with the seagrass, Thalassia testudinum, in the laboratory and field

The structure of a macrobenthic invertebrate community associated with the seagrass, Thalassia testudinum, was evaluated under laboratory and field conditions. The research focused on: (1) the effects of pollution stress from a representative drilling fluid used in offshore oil and gas operations, and (2) a comparison of responses of the seagrass-invertebrate community in the laboratory and field. The numbers of macrobenthic invertebrates were suppressed by drilling fluid at both exposure periods in the laboratory, but inhibitory effects were absent in the field. Invertebrate densities in the field were similar among control and treated plots, and were much lower than densities occurring in the laboratory control. In most instances, species richness values were similar in the field and laboratory at the end of each 6 and 12 week period.

Use of artificial sediments in a comparative toxicity study with larvae and postlarvae of the grass shrimp, Palaemonetes pugio

An artificial sediment was tested for use in evaluating the potential hazard of toxicants on benthic organisms. The seawater-sediment system was assessed by use of the pyrethroid insecticide, fenvalerate, as the model toxicant for testing with larvae of the grass shrimp, Palaemonetes pugio, an ecologically important estuarine species. The sediment was prepared from commercially available components, and mixed with the toxicant to provide concentrations of 1, 10 and 100 microg fenvalerate kg(-1) dry sediment in 20 ppt seawater. Sediment free of the insecticide served as the control. Throughout the study, fenvalerate was not detected in the water column, but was measured in sediment at the nominal concentration of 100 microg kg(-1). The P. pugio population was adversely affected by fenvalerate. The effect occurred at metamorphosis, when larvae changed from pelagic individuals to benthic organisms. At this period, larvae were in direct contact with sediment. A portion of the population was tolerant of the insecticide.

Wetland plant seedlings as indicators of near-coastal sediment quality: interspecific variation

The toxicities of whole sediments collected from an urbanized Florida bayou-estuary were determined for the epibenthic mysid, Mysidopsis bahia, and the infaunal amphipod, Ampelisca abdita. In addition, the phytotoxicities of the same sediments were evaluated using rooted macrophytes, Scirpus robustus Pursh and Spartina alterniflora Loisel. The results of the 24 bioassays conducted for 7-28 days, were compared for interspecific differences. Toxicity to the benthic invertebrate species was predicted to be likely in all cases, based on comparison to effects-based sediment quality assessment guidelines. However, acute toxicity was uncommon and occurred at only one of the six sampling stations. In contrast, several sediments were either significantly phytostimulatory or phytoinhibitory (P < 0.05) relative to a reference estuarine sediment. The results show the importance of considering interspecific variation and phytoassessment in contaminated sediment evaluations. Phytoassessment, the focus of this research, provided useful information that would have been missing if only the animal test species were used. For this reason, the phytotoxicity data base needs to be increased to better evaluate the relevance of most current contaminated sediment evaluations for coastal areas and also to determine the realism of proposed numerical sediment quality assessment guidelines and criteria.