Back-pumping of agricultural runoff into a large shallow lake and concurrent changes in the macroinvertebrate assemblage.

Macroinvertebrates were monitored during the summer of 2001 at two paired reference and impact sites, to assess potential effects of agricultural runoff, which was back-pumped upstream into Lake Okeechobee, USA. Monitoring was conducted prior to back-pumping (Pre, April-May), during back-pumping (BP, June-September) and during the following year (Post, April-September, 2002, impact sites only). Prior to back-pumping, the macroinvertebrate communities at both impact sites differed from those at their respective reference sites. During back-pumping, differences in the macroinvertebrate communities were slightly less pronounced among the eastern pair of sites but were more pronounced at the western sites than they were prior to back-pumping. There also were marginal differences in the macroinvertebrate communities at both impact sites between the Pre and BP periods, while very little change was observed in the communities among the same periods at both reference sites. For the Post-Pre and Post-BP period comparisons, there were clear differences between the macroinvertebrate communities at the western impact site, while less pronounced differences were observed at the eastern impact site. This suggests that the Post back-pumping assemblage did not return to that observed prior to back-pumping activity. During BP, several water-quality variables appeared to be different among each of the paired sites and the correlation between water-quality variables and macroinvertebrate community structure was strong at the western sites but weaker at the eastern sites. This suggests that a combination of abiotic variables may have been influencing the community structure at the western sites while back-pumping was occurring. Macroinvertebrates at all sites reflected poor water quality, but more taxonomic changes during back-pumping were observed at the impact sites than at the reference sites. It is not known if these taxonomic changes resulted in impacts among the macroinvertebrate community or to higher-trophic-level predators such as fish. If lake restoration activities result in a shift to a less pollution-tolerant macroinvertebrate community, and the anticipated reduction in future back-pumping activity does not occur, the macroinvertebrate community may return to one that is dominated by pollution-tolerant taxa in affected portions of the lake.

Large-scale mapping and predictive modeling of submerged aquatic vegetation in a shallow eutrophic lake.

A spatially intensive sampling program was developed for mapping the submerged aquatic vegetation (SAV) over an area of approximately 20,000 ha in a large, shallow lake in Florida, U.S. The sampling program integrates Geographic Information System (GIS) technology with traditional field sampling of SAV and has the capability of producing robust vegetation maps under a wide range of conditions, including high turbidity, variable depth (0 to 2 m), and variable sediment types. Based on sampling carried out in August-September 2000, we measured 1,050 to 4,300 ha of vascular SAV species and approximately 14,000 ha of the macroalga Chara spp. The results were similar to those reported in the early 1990s, when the last large-scale SAV sampling occurred. Occurrence of Chara was strongly associated with peat sediments, and maximal depths of occurrence varied between sediment types (mud, sand, rock, and peat). A simple model of Chara occurrence, based only on water depth, had an accuracy of 55%. It predicted occurrence of Chara over large areas where the plant actually was not found. A model based on sediment type and depth had an accuracy of 75% and produced a spatial map very similar to that based on observations. While this approach needs to be validated with independent data in order to test its general utility, we believe it may have application elsewhere. The simple modeling approach could serve as a coarse-scale tool for evaluating effects of water level management on Chara populations.

A comparison of three methods to collect submerged aquatic vegetation in a shallow lake.

Two boat-based and one in-water sampling method have been used to collect submerged aquatic vegetation (SAV) as part of a long-term monitoring program in Lake Okeechobee, Florida, USA. The boat-based methods consisted of collecting SAV with a ponar dredge, used only to collect Chara, and an oyster tongs-like rake apparatus, used to collect all SAV. The in-water method involved use of a 0.5 m2 PVC quadrat frame deployed by a diver. During summer 2002, SAV biomass samples were collected using all three methods at various sites in the lake to compare between-methods sampling precision. Sites used for these comparisons were selected based on plant type, plant density and sediment type. Statistical comparisons indicated that there were significant (p < or = 0.05) biomass differences in 8 of 15 possible pairwise comparisons between sampling method biomass means. In four of the eight comparisons, significantly higher biomass mean values were obtained using the quadrat frame. In three of the remaining four comparisons, significantly higher biomass mean values were obtained with the rake apparatus. For the fourth comparison, a significantly higher biomass mean value was obtained with the ponar dredge. Three of the four relationships between SAV biomass collected by the rake and the quadrat and the rake and the quadrat/ponar dredge were statistically significant, linear and explained between 67 and 78% of the biomass variability. There were no significant differences between regression coefficients or elevations for these relationships, therefore these regressions estimated the same population regression. The population regression coefficient was 0.95, suggesting that the quadrat and ponar over-sampled relative to the rake, but the amount of this over-sampling was very small. Since there was no consistent pattern in the sampling method which yielded the significantly different biomass values and there were no significant differences in sampling precision across a range of plant species, plant densities and two sediment types, the boat-based rake method appears to be a suitable replacement for the previously used ponar dredge and quadrat methods, when in-lake measurements are not practical.