Impacts of intensive row-crop agriculture in lakes that receive runoff have been well-documented. Light limitation and eutrophication resulting from elevated suspended sediment loads and associated nutrients (nitrogen and phosphorus) directly affect lake productivity. Agricultural conservation practices have demonstrated long-term improvements in lake water quality. However, ecosystem resilience following extreme weather events in these improved systems has not been well documented. In March 2016, a series of storm events occurred in North Mississippi producing over 203 mm of rainfall during a 6-day period and causing major flooding in this highly cultivated region. A paired watershed approach was used to assess lake resilience during this extreme event. Beasley Lake watershed comprising 624 ha has about 39% of the watershed in varying conservation practices with 106.6 ha (17%) in structural conservation practices (e.g., vegetated buffers, constructed wetlands) with an additional 135.4 ha (22%) in reduced tillage. In contrast, Roundaway Lake comprising 1698 ha has only 46 ha (about 2.7%) in structural practices similar to Beasley Lake. Lake total suspended sediment (TSS) concentration, water transparency (as Secchi depth), total phosphorus (TP), total nitrogen (TN), and water column chlorophyll a concentration was measured in both lakes before and after flooding.
Results/Conclusions
Water transparency was clearest in the watershed with the most conservation practices (Beasley Lake) when TSS concentrations were lowest before (p = 0.008) and after (p = 0.002) flooding. Beasley TSS peaked at 202 mg/L during flooding whereas Roundaway often had higher TSS before (339-1012 mg/L) and after flooding (144-430 mg/L). Pre-flood TP concentrations were lowest (p = 0.012) in Beasley Lake (85-230 μg/L) relative to Roundaway (263-394 μg/L). However post-flood spring TP was comparable (p = 0.094) between Beasley Lake (104-358 μg/L) and Roundaway (136-472 μg/L) while post-flood summer TP was significantly lower (p = 0.028) in Beasley (49-132 μg/L) than Roundaway (46-252 μg/L). TN concentrations were unaffected by flooding in both watersheds. However, Beasley Lake had significantly lower TN (921-2052 μg/L) than Roundaway (1031-5570 μg/L) (p = 0.011). Coinciding with water transparency, pre-flood algal biomass (as chlorophyll a) was greater in Beasley (14-35 μg/L) than in Roundaway (8-10 μg/L) (p = 0.009) and recovered earlier after flooding in Beasley Lake (10 weeks post-flood) than in Roundaway (14-16 weeks post-flood). Watersheds with more conservation practices provide greater resilience after extreme flooding events by mitigating light limitation and eutrophication while more quickly recovering to pre-flood conditions.