Lake Pepin Site Specific Eutrophication Criteria

Lake Pepin was assessed for "nutrient impairment" as a part of the 2002 303(d) assessment. Since numeric lake eutrophication criteria were not available at that time, ecoregion-based numeric translators were used to interpret the narrative standards that referred to excess algal growth and associated impairment. Lake Pepin was assessed based on the following data collected between June through September from 1991-2000: total phosphorus (TP) =198 (±4) μg/L (n=160), chlorophyll-a (Chl-a) = 25 (±1) μg/L (n=158), and Secchi= 1.0 (±0.3) m (n= 240). Since there were no specific translators for the ecoregion where Lake Pepin was located (Driftless Area), translators from the adjacent two ecoregions that comprise much of Pepin's watershed were used in the assessment. Based on the assessment Lake Pepin's TP was well above the North Central Hardwoods Forest (CHF) and Western Corn Belt Plains (WCP) thresholds, while chlorophyll-a and Secchi exceeded CHF thresholds. As a result, Pepin was placed on Minnesota's 2002 303(d) list. Though Lake Pepin is a natural lake, its characteristics are reservoir-like and as such, it is presumed to require site-specific criteria, as noted in guidance. A central task of the Lake Pepin Total Maximum Daily Load (TMDL) is development of site-specific criteria for chlorophyll a (Chl-a), transparency and phosphorus concentration that provides adequate protection of aquatic recreational use. This task has evolved over time as more knowledge has been gained on Lake Pepin and its interrelationship with upstream navigation pools and the major tributaries that drive the overall system. Recognizing the complexities and linkages of Pepin, upstream navigational pools and major tributaries the Lake Pepin TMDL Science Advisory Panel (SAP) recommended that MPCA develop eutrophication standards for Lake Pepin and Pools1-8. The SAP further acknowledged that these waterbodies differ sufficiently from typical lakes and rivers to warrant site specific standards. They recommended that Lake Pepin eutrophication standards should be integrated into the statewide river eutrophication criteria development. A detailed analysis of river and navigational pool data was conducted in support of this request (Heiskary and Wasley 2010b) and that effort complements and builds on analysis presented in this current report, which will focus on Lake Pepin. Likewise, detailed analysis, conducted as a part of the South Metro Mississippi turbidity TMDL (which includes upper Lake Pepin), demonstrated that suspended sediment is a primary limiter of Secchi transparency in Pepin and that TMDL was selected as the primary basis for addressing Secchi transparency. The recently promulgated lake eutrophication standards allow for the development of site specific criteria for reservoirs as noted in Minnesota Rules Chapter 7050 (2008) (https://www.revisor.leg.state.mn.us/rules/?id=7050.0222 ). Minnesota's "Lake Nutrient TMDL Protocols and Submittal Requirements" (MPCA 2007) provides a framework for establishing a site-specific water quality standard and pertinent information that should be considered (e.g. pp 79-83; http://www.pca.state.mn.us/publication /wq-iw1-10.pdf ). This current report was prepared in support of the development of site-specific eutrophication criteria for Lake Pepin. The report includes: • Basic background information on Lake Pepin and previous efforts to establish goals for the lake; • An up-to-date analysis of data for the lake, which focuses on the 22 years of data (1985-2006) used in the development of the Upper Mississippi River-Lake Pepin (UMR-LP) model and recent data that has been collected by the Long Term Resource Monitoring Program (LTRMP) for the period 2006-2009; • Analysis of data from recent low-average flow years to further describe relationships between the Mississippi River and upper and lower segments of Lake Pepin; • Review of model predictions for various years and reduction scenarios that contribute to criteria development; and • Summary of proposed site specific criteria for Lake Pepin. This report and two reports that describe criteria setting for the navigation pools and major river inflows to the Mississippi river above Lake Pepin (Heiskary and Wasley 2010) and statewide river eutrophication criteria development (Heiskary et al. 2010) provide the technical basis for Lake Pepin's site specific criteria. Based on all three reports TP and Chl-a criteria of 100 μg/L and 28 μg/L, respectively, are proposed for Lake Pepin. These criteria provide protection of aquatic recreational uses for Lake Pepin and the downstream pools and should be applicable over the range of flows for which the criteria were developed. The Lake Pepin criteria need to be considered in a Mississippi River context in the following manner: it is through phosphorus and chlorophyll reductions upstream of Lake Pepin, in specific watersheds (Lower Minnesota River, Crow River, Sauk, etc) that the Lake Pepin criteria are most likely to be achieved. The proposed Lake Pepin criteria should not be used in isolation to imply that P reductions anywhere upstream of the lake will have the desired impact. The main biological activity affecting Lake Pepin trophic status is not taking place in the lake, but upstream of it. In particular, reductions upstream of the MCES's WWTPs will be needed to achieve the desired standards. The Lake Pepin criteria are not stand-alone goals to be pursued in isolation. Rather, they belong to a set of goals (criteria) for the Mississippi River system, which, if pursued systematically in unison, will achieve the desired results.