Nutrients and sediment in the Green Lake marshes

Green Lake is the deepest natural inland lake in Wisconsin. Located in the Southeastern Wisconsin Till Plains ecoregion, its watershed is dominated by agriculture. Many of the lake’s primary tributaries have excess total phosphorus (TP) and suspended solids (SS) which degrade water quality within the lake. Many of these impaired streams, however, flow through large marshes which presents an opportunity for nutrient and sediment reduction before flows reach the lake itself. The largest marsh, Silver Creek Estuary (SCE) has been a clear water macrophyte-dominated system since around 2006. Prior to carp exclusion and a reduction in an upstream TP point source, the marsh was turbid and phytoplankton-dominated. U.S. Geological Survey (USGS) monitoring data at the outlet of the marsh (1987 – 2017) were used to determine whether the shift from a turbid to clear state decreased TP and SS loading to Green Lake. USGS monitoring data at the primary marsh inlet and outlet (2012 – 2017) were used to quantify when the macrophyte-dominated marsh retained or was a source of TP and SS. Samples taken at the marsh inlet and outlet by the USGS and as part of this study (2006 – 2017) were also compared to determine if nutrient concentrations decreased through the marsh while it was macrophyte dominated.

In all seasons, TP concentrations at the outlet of SCE decreased following the equilibrium shift and the reduction of the upstream point source. The SS concentrations only decreased in spring and summer. The biological shift likely contributed to the SS reductions and TP reductions in the spring and summer. The TP reductions observed beyond the growing season were likely due to the TP reduction from the upstream point source. Over the five-year period (2012 –2017), the restored marsh retained both TP and SS. Seasonally, SS was exported in the fall, but more than that amount was retained in all other seasons resulting in the marsh being a SS sink. In summer (2006 – 2017), TP, total dissolved P (TDP), SS, and total nitrogen (TN) concentrations all decreased from the inlet to the outlet of the marsh.

The second marsh, County K Marsh (CKM), is believed to be similar to SCE before restoration. Efforts to exclude carp and establish macrophytes began in 2015, with intentions to shift the marsh from phytoplankton to macrophyte-dominance. TP and SS loads from the outlets of the macrophyte-dominated (SCE) and phytoplankton-dominated (CKM) marshes were compared across seasons using USGS monitoring data from 2012 – 2017. Inlets and several locations within each marsh were also sampled regularly over two field seasons (July – October 2016 and April – October 2017) to assess how nutrient reductions differed within the macrophyte versus phytoplankton systems. TP, total suspended solids (TSS) and TN loads were also estimated for each marsh inlet and outlet and used to quantify marsh storage for each sampling event.

Flow-weighted mean TP and SS concentrations at the two marsh outlets were similar in the winter but increased more during the growing season in the phytoplankton-dominated marsh than in the macrophyte-dominated marsh. Fall TP concentrations in CKM remained high compared to SCE. Summer SS concentrations were also higher in CKM compared to SCE. Compared to all 12 sampling sites, both marshes had inputs with high concentrations of TP, TDP, TSS and TN. Interior sites of CKM, however, had higher TP and TSS concentrations than the inlets. TP, TDP, TSS and TN all decreased downstream through SCE. In CKM, only TN and TDP decreased from inlets to downstream. In total over 11 sampling events, CKM was estimated to have retained TN but not TP or TSS while SCE retained all three. Equilibrium phosphorus concentrations (EPCo), the ambient P concentration that determines whether sediments absorb or release P, estimated for each marsh suggested that SCE sediment may release more P than CKM sediment. On the day of sampling, water column DRP concentrations indicated sediments in both marshes were releasing P. Since the daily storage estimates did not suggest internal P loading for SCE, P released from sediment was likely deposited elsewhere in the marsh and not exported to Green Lake.

Similar winter nutrient and sediment concentrations measured in both marshes but very different summer concentrations suggest biological processes are instrumental during the growing season. While an equilibrium shift for CKM would not be accompanied by a stark reduction in an upstream TP source, as was the case for SCE, data suggest that excluding carp from and introducing more macrophytes to CKM could reduce TP and SS export to Green Lake.