Assessing drivers of variability in the chlorophyll phosphorus relationship in Wisconsin lakes CO_06

Purpose

We will perform targeted sampling of a subset of Wisconsin lakes that are identified as having particularly high or low chlorophyll response to phosphorus concentrations in order to identify the primary driver(s) of these phenomena. These lakes are currently part of the CLMN and were included in the dataset utilized by Diebel to develop Wisconsin’s phosphorus standards, and appeared as outliers in that analysis. Sampling will be performed throughout the 2013 ice-free season, with the goal of accurately characterizing seasonal trends in water physical and chemical properties, with an emphasis on inflows/outflows, nutrient dynamics, and plankton community composition. The hydrologic connectivity, watershed characteristics, macrophyte data, and fish community data in each lake will also be compiled to facilitate cross-lake comparisons.

Objective

Phosphorus numeric criteria are an important factor in impaired waters (303d) identification. However, a number of factors can influence the response of a particular lake or reservoir to a given phosphorus load or concentration. Improved understanding of these factors will facilitate identification of water bodies for which current numeric criteria are likely to be either too stringent or insufficiently protective, and potentially guide the development of site-specific criteria in these cases. For this project, we will focus on shallow lakes and reservoirs with total phosphorus (TP) concentrations between 20 and 50 micrograms per liter, as these systems show a particularly large degree of variability in the frequency of nuisance algal blooms relative to phosphorus concentrations. Out of more than 60 candidate sites (those used in developing the phosphorus standard for shallow lakes and falling in the 20-50 microgram TP range), we will systematically select six study sites (three high and three low response lakes). While further work is required to select the optimal six sites (SITES TBD), preliminary analysis suggests that all sites will be natural lakes located in the Northern Lakes and Forest region. Each of these lakes will be sampled five times throughout the 2013 ice-free season. Basic limnological data (temperature profile, etc.) will be collected on site, and samples will be collected for phosphorus (TP, TDP, and SRP) and nitrogen (TN, nitrate+nitrite, ammonia) analysis at the State Lab of Hygiene. Net tows will also be collected for phytoplankton and zooplankton enumeration, which will be performed by Science Services staff.

Outcome

Outcome: This study will culminate in a report that provides quantitative information on driver(s) of “atypical” response to phosphorus concentrations in shallow lakes that will guide the development site-specific P criteria. Sample sites: 6 Sampling events: 3 in FY 2013 and 2 in FY 2014 Sampling will be complete by 10/31/2013; sampling will be evenly spaced throughout the ice-free season All field data will be entered into SWIMS within two weeks of sampling (i.e., all data will be entered by 11/15/2013). Data will be entered by McDonald and Diebel. A final project report will be completed by the end of FY 2014.

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