Nutrient Uptake and Short-Term Responses of Phytoplankton and Benthic Algal Communities from a Subarctic Pond to Experimental Nutrient Enrichment in Microcosms

Climate warming is anticipated to affect high-latitude regions, including abundant ponds of the Hudson Bay Lowlands (HBL). However, it remains unclear if associated increased frequency of nutrient pulses will be rapidly consumed by aquatic biota and sediment or lead to a rise in pond-water nutrient concentrations. Here, we performed a nutrient-amendment experiment to examine short-term (<= 72 h) nutrient uptake and identify the consumers of the added nutrients (planktonic vs. benthic communities). Microcosms (1 L) with and without sediment were experimentally amended with in-organic nitrogen (nitrate, ammonium) with and without phosphate. Amended nitrate and ammonium concentrations remained high in microcosms without sediments, and phytoplankton biomass did not change relative to the un-amended control. However, phosphate concentration declined significantly in microcosms without sediment, resulting in significant increase of phytoplankton biomass after 72 h. In the presence of sediment, amended nutrients were rapidly removed from the water, stimulating benthic algal biomass when phosphate was co-amended with ammonium or nitrate. Phytoplankton biomass was significantly elevated in microcosms with sediment compared to those without sediment, regardless of whether nutrients were amended or not, indicating that sediment and associated benthic biofilm stimulate phytoplankton growth, likely via supply of nutrients to the overlying water column. A key outcome of the experiment is that pulsed nutrients were taken up rapidly and primarily by the benthic community. Findings suggest that shallow ponds in the HBL are capable of rapidly consuming pulsed nutrient supplies, as may occur due to hydroclimatic events, climate warming and other disturbances.