Algal Demand Drives Sediment Phosphorus Release in a Shallow Eutrophic Cove

Algae play an important role in the internal nutrient cycling of shallow lakes and coves. Algae of shallow water bodies have been shown to thrive and even lead to eutrophic conditions despite a lack of measurable quantities of biologically available phosphorus (P) in the water during summer months. To study how sediment P release and algal growth are connected, water and sediment samples were collected in a shallow eutrophic cove on Beaver Lake in northwest Arkansas. Water quality profiles depicting temperature, dissolved oxygen, nutrients, metals, and photic zone chlorophyll-a were collected weekly from 21 May to 10 July 2018 at three points in the cove: one shallow, one at the cove midpoint, and one at the deepest part of the cove. Cove sediment samples were collected at similar points as the water quality samples for equilibrium P concentration (EPCo) analysis, sediment core incubation, and sediment composition. EPCo for the sediments ranged from 0.024 to 0.027 mg L-1. Sediment cores exposed to aerobic conditions typical of shallow areas had P release rates ranging from 1.37 to 2.02 mg m-2 d-1. Concentrations of soluble reactive P (SRP) in the water column from the weekly water quality sampling averaged 0.002 ±0.003 mg L-1, and photic zone SRP concentrations averaged 0.002 ±0.004 mg L-1 for all sampling sites. The chlorophyll-a concentration increased from 10 to 40 µg L-1 during the period from 21 May to 25 June. When SRP << EPCo, conditions favor the release of SRP from sediments to the overlying water. This was confirmed with the aerobic sediment core incubation in which algal demand was controlled using dark conditions and the release rates were >1.37 mg L-1 d-1. Core aerobic release rates and EPCo conditions both confirmed the release of P under aerobic conditions; however, it appears that algal demand sustained low SRP conditions. This created a nutrient cycle in which algae imposed a nutrient gradient favoring P release by keeping SRP conditions below the EPCo. This study indicates that algal growth potential in shallow water bodies is not limited by SRP concentrations measured within the water column. Studies of shallow water bodies with low SRP concentrations and high productivity should look to the sediments as a source of P to fuel algal growth. Finally, these findings suggest that coves play an integral part in algal production and should not be overlooked when determining the overall P budget for a lake or reservoir. Keywords: Aerobic phosphorus release, Equilibrium phosphorus concentration, Sediment core incubation.

Use of the [14C]Leucine Incorporation Technique To Measure Bacterial Production in River Sediments and the Epiphyton

ABSTRACT Bacterial production is a key parameter for the understanding of carbon cycling in aquatic ecosystems, yet it remains difficult to measure in many aquatic habitats. We therefore tested the applicability of the [14C]leucine incorporation technique for the measurement of bulk bacterial production in various habitats of a lowland river ecosystem. To evaluate the method, we determined (i) extraction efficiencies of bacterial protein from the sediments, (ii) substrate saturation of leucine in sediments, the biofilms on aquatic plants (epiphyton), and the pelagic zone, (iii) bacterial activities at different leucine concentrations, (iv) specificity of leucine uptake by bacteria, and (v) the effect of the incubation technique (perfused-core incubation versus slurry incubation) on leucine incorporation into protein. Bacterial protein was best extracted from sediments and precipitated by hot trichloroacetic acid treatment following ultrasonication. For epiphyton, an alkaline-extraction procedure was most efficient. Leucine incorporation saturation occurred at 1 μM in epiphyton and 100 nM in the pelagic zone. Saturation curves in sediments were difficult to model but showed the first level of leucine saturation at 50 μM. Increased uptake at higher leucine concentrations could be partly attributed to eukaryotes. Addition of micromolar concentrations of leucine did not enhance bacterial electron transport activity or DNA replication activity. Similar rates of leucine incorporation into protein calculated for whole sediment cores were observed after slurry and perfused-core incubations, but the rates exhibited strong vertical gradients after the core incubation. We conclude that the leucine incorporation method can measure bacterial production in a wide range of aquatic habitats, including fluvial sediments, if substrate saturation and isotope dilution are determined.