A Statewide Assessment of Drivers Influencing Biotic Integrity in Perennial, Wadeable Prairie Streams of South Dakota

Biological assessments are used to evaluate the biotic integrity of a system. Great Plains prairie streams are harsh environments due to variable climate, topography, vegetation, and human development. This presents a challenge to water resource managers who must detect impairment and determine causation. The objectives of this study were: 1) to develop a macroinvertebrate Index of Biotic Integrity (IBI) for the Northwestern Great Plains (NWGP) and compare it to an existing Northern Glaciated Plains (NGP) IBI, and 2) to identify and evaluate statewide abiotic drivers of biotic integrity. The IBIs were hypothesized to differ regionally, as were abiotic drivers explaining their variation. Physical, chemical, and biological data were collected at 65 perennial, wadeable stream sites in the NWGP. Biological metrics were calculated and then optimized using six statistical screening tests. Random forest modeling, cluster analysis, and nonmetric multidimensional scaling (NMDS) were employed to evaluate abiotic drivers. Final IBI metrics in the NWGP were family richness, richness of non- Insecta, richness of collector-filterers, and richness of Plecoptera, Odonata, Ephemeroptera, and Trichoptera. IBI scores ranged between 3 and 93 (x̅ = 37) and successfully differentiated between least impaired and most impaired sites (Kruskal- Wallis ANOVA p \u3c 0.01). All metrics comprising the NWGP and NGP IBIs differed. The NWGP IBI yielded a similar range of scores to the NGP IBI (range: 24 to 100, x̅ = 58). No significant differences were found in IBI scores among level IV ecoregions of either the NWGP or NGP. Random forests identified local physical and chemical drivers of biotic integrity and watershed-scale drivers of in-stream abiotic conditions. Major drivers of IBI scores included salinity, specific conductance, fine and gravel substrates, total riffle length, and stream discharge (46% of variation explained). Each component IBI metric was related to a unique combination of drivers (17 to 56% of variation explained). Ecoregion (level III and level IV) and river basin variables generally improved model performance for local abiotic drivers (x̅ = 9% increase in variation explained). Cluster analysis groupings largely separated NGP and NWGP streams, while NMDS ordinated groups along a land use gradient differentiating level III ecoregions. Human use, pasture and hay, and nitrogen loading were more positively associated with NGP groups, while total and riparian herbaceous cover and turbidity were more positively correlated with NWGP groups. This study developed the first South Dakota ecoregion-wide IBI west of the Missouri River, and greatly expanded geographic coverage and use of the IBI in making impairment decisions. These results can also be incorporated in causal analysis of impairment

Growing Season Productivity and Trophic Classification of Oak Lake, Brookings County, South Dakota

Changes in lake trophic state present concerns to water resource managers interested in maintaining water quality to support assigned beneficial uses. Contemporary methods of classifying lakes involve the use of surrogate indicators of production. However, some of these measurements are sensitive to wind induced resuspension of sediments, leading to inflated indications of basin production. This source of error is common to many shallow glacial lakes in eastern South Dakota and southwestern Minnesota. The objectives of this effort were to (1) estimate and define the trend in seasonal water column net and gross primary production and community respiration within a shallow pothole basin, (2) compare the mean net primary productivity values among three sub-basin sites and (3) evaluate trophic state classification using surrogate measures against actual production measurements. Water production as measured at three basin sites in Oak Lake, South Dakota, was evaluated using the light/dark bottle method once every two weeks throughout the 2010 growing season. Mean net primary productivity was 741 mg C•m-2•d-1and ranged from 35 to 1,462 mg C•m-2•d-1. Estimated to the light compensation depth, Oak Lake mean net primary production would lead to a eutrophic classification for this basin but would range between mesotrophic and hypereutrophic throughout the growing season. Trophic State Index values, derived from Secchi depth, ranged between 65 and 83, with a mean of 75, leading to an index classification of eutrophic or hypereutrophic. Secchi transparency explained 82% of the variation in net primary production while chlorophyll a explained only 17%. We concluded that Secchi transparency is an adequate surrogate for planktonic production despite consistently overestimating actual production levels within this basin

Relationships Between Net Primary Production, Water Transparency, Chlorophyll A, and Total Phosphorus in Oak Lake, Brookings County, South Dakota

Lake trophic state is of primary concern for water resource managers and is used as a measure of water quality and classification for beneficial uses. Secchi transparency, total phosphorus and chlorophyll a are surrogate measurements used in the calculation of trophic state indices (TSI) which classify waters as oligotrophic, mesotrophic, eutrophic or hypereutrophic. Yet the relationships between these surrogate measurements and direct measures of lake productivity vary regionally and may be influenced by external factors such as non-algal turbidity. Prairie pothole basins, common throughout eastern South Dakota and southwestern Minnesota, are shallow glacial lakes subject to frequent winds and sediment resuspension. Light-dark oxygen bottle methodology was employed to evaluate vertical planktonic production within an eastern South Dakota pothole basin. Secchi transparency, total phosphorus and planktonic chlorophyll a were also measured from each of three basin sites at biweekly intervals throughout the 2012 growing season. Secchi transparencies ranged between 0.13 and 0.25 meters, corresponding to an average TSISD value of 84.4 (hypereutrophy). Total phosphorus concentrations ranged between 178 and 858 ug/L, corresponding to an average TSITP of 86.7 (hypereutrophy). Chlorophyll a values corresponded to an average TSIChla value of 69.4 (transitional between eutrophy and hypereutrophy) and vertical production profiles yielded areal net primary productivity values averaging 288.3 mg C∙m-2∙d-1 (mesotrophy). Our results support the hypothesis that resuspended non-algal turbidity, not planktonic production, decreases water transparency and reduces potential net primary production. Chlorophyll a TSI values corresponded most closely with measurements of planktonic production and better represented the trophic state of this basin