Episodic Nutrient Addition Affects Water Column Nutrient Processing Rates in River‐to‐Lake Transitional Zones

Abstract

In the Laurentian Great Lakes, nonpoint sources contribute most of the total nutrient load delivered to eutrophic embayments (Maccoux et al., 2016;Robertson & Saad, 2011). However, the export of nutrients from many nonpoint sources (e.g., agriculture) is temporally variable and often associated with water runoff from precipitation events (Withers & Jarvie, 2008). Although nonpoint sources of nutrients contribute to the overall enrichment of surface wa-Abstract Storm-driven nutrient loading from tributaries can fuel eutrophication in nearshore and open water areas of lentic ecosystems. However, nutrient processing in river-to-lake transitional zones can substantially alter the amount and composition of nutrients transported to lakes from upstream surface waters. We measured the removal of nutrients and dissolved organic carbon (DOC) from the water column in the Fox rivermouth (Green Bay, Lake Michigan) to evaluate the response of rivermouth plankton to episodic nutrient enrichment. Light and dark water column incubations (8-12 hr) were conducted on four occasions from April through September to measure changes in dissolved nitrogen (N), phosphorus (P), and DOC concentrations in three locations along the Fox rivermouth. Two incubation experiments were conducted on consecutive days, (a) under ambient nutrient concentrations, and (b) under experimentally enriched N and P concentrations. Spatial and temporal variation was observed in nutrient uptake rates, but light incubations consistently had higher nutrient uptake rates than dark incubations. Nutrient enrichment increased total dissolved P and total dissolved N uptake and DOC release in light incubations, but only increased total dissolved P uptake in dark incubations. Moreover, nutrient uptake ratios (N:P) decreased from ambient to nutrient enriched conditions and indicated preferential P uptake by phytoplankton communities in light conditions. Our study substantiates that rivermouths can process nutrients bound for downstream ecosystems and demonstrates the potential of plankton communities to dynamically increase net uptake rates in response to episodic nutrient enrichment.Plain Language Summary Nutrients washed into lakes from human activities (e.g., fertilizer application and livestock waste) can trigger the excessive growth of algae and harm nearshore ecosystems. Areas where large rivers enter lakes, also known as rivermouths, can capture these nutrients coming from the landscape and make them less available to algae in nearshore ecosystems. Yet, nutrients washed into rivermouths from rain events are only present for a short period of time and we do not know how much of these brief nutrient inputs can be removed through biological processes. Our study compared biological nutrient uptake rates in Fox rivermouth (Green Bay, Lake Michigan) under background conditions to those that mimicked a rapid increase in nutrients from a rain event. We found that biological nutrient uptake immediately increased when nutrients were added to the water column and changed the b...

Effects of a Storm on the Transformation and Export of Phosphorus Through a Subtropical River‐Turbid Estuary Continuum Revealed by Continuous Observation

“…(2007) found that there is a slow but steady increase in the phytoplankton standing stock over time in Kaneohe Bay after storm pulse input. Similar processes occur at locations where major rivers flow into lakes (Pearce et al., 2021), but such locations are not dominated by SPM and thus result in nutrient uptake by phytoplankton, including preferential P uptake and increased eutrophication in these lake estuaries.…”

“…Such changes are typical of such systems, for example, De Carlo et al (2007) found that there is a slow but steady increase in the phytoplankton standing stock over time in Kaneohe Bay after storm pulse input. Similar processes occur at locations where major rivers flow into lakes (Pearce et al, 2021), but such locations are not dominated by SPM and thus result in nutrient uptake by phytoplankton, including preferential P uptake and increased eutrophication in these lake estuaries. Lin et al (2022) determined the concentration and flux of N species in the river-estuary continuum for the same storm as has been studied in detail here for P species.…”

Wood Increases Greenhouse Gas Emissions and Nitrate Removal Rates in River Sediments: Risks and Opportunities for Instream Wood Restoration

“…Furthermore, transformation of gases in the water column or headspace of the microcosm could modify the relative concentrations of GHGs actually emitted from sediments before sampling, for example, oxidation of CH 4 to CO 2 . Other potential limitations include the irregular spiking to high NO 3 – concentrations (atypical of most systems in reality), which could also affect microbial assemblage dynamics, though this could represent storm-driven high nutrient loading . Furthermore, high concentrations of DOC and associated metrics in treatments without sediment are not at all representative of instream wood in the surface water column of streams because, in these circumstances, DOC would be quickly transported .…”

“…72 Other potential limitations include the irregular spiking to high NO 3 − concentrations (atypical of most systems in reality), which could also affect microbial assemblage dynamics, though this could represent stormdriven high nutrient loading. 66 Furthermore, high concentrations of DOC and associated metrics in treatments without sediment are not at all representative of instream wood in the surface water column of streams because, in these circumstances, DOC would be quickly transported. 19 While these artifacts of the experimental design may prevent reliable inference and scaling of metrics to absolute values, the broad patterns observed here are likely to be somewhat representative of in situ observations.…”

Periphyton Phosphorus Uptake in Response to Dynamic Concentrations in Streams: Assimilation and Changes to Intracellular Speciation

“…Recent work has highlighted the potential of benthic periphyton to accumulate excess P in oligotrophic streams in response to short duration pulses of elevated P concentration . Accumulation of excess P under conditions of high availability (i.e., luxury uptake) has also been observed in marine and freshwater phytoplankton − in addition to benthic algae. − Luxury uptake is achieved through intracellular conversion and storage of inorganic P molecules as polyphosphate polymers . Polyphosphate accumulation may therefore serve as an underappreciated mechanism of transient P retention in streams that receive short duration high concentrations inputs of P from overland flows or episodic point sources.…”