Do mobile consumers homogenize the distribution of resources in stream food webs? A test with overlapping fish and mussel aggregations

Abstract

In streams, unionoid mussels and fish form aggregations that exert bottom‐up and top‐down effects on food webs, but the magnitude and spatial extent of their effects are controlled by species traits. Sedentary mussels live burrowed in the sediment in patchily distributed dense aggregations (mussel beds) where they filter seston and provide a local, relatively constant nutrient subsidy. In contrast, fish move on and off mussel beds, and thus comprise a transient nutrient subsidy. We asked how overlap between fish and mussels influences nutrient recycling and resource distribution in streams. We conducted an 8‐week study in experimental streams where we created mussel beds (comprised of two species, Actinonaias ligamentina and Amblema plicata), manipulated the occurrence of a grazing minnow (Campostoma anomalum), and tracked nutrient (nitrogen and phosphorus) and resource (algae, detritus, and chironomids) abundance up and downstream of the mussel beds. In general, neither consumer had strong effects on the concentration or spatial distribution of nutrients. Water turnover time in our experimental streams may have diluted fish and mussel nutrient excretion effects, making it difficult to detect spatial patterns during a given sampling period. Fish controlled the abundance and productivity of algae. In treatments without fish, large mats of filamentous algae formed early in the experiment. These algae senesced, decomposed, and were not replaced. When fish were present, algae consisted of attached biofilms with consistent biomass and spatial distribution over time. Although previous work has shown that mussels can have strong, seasonal bottom‐up effects on both primary and secondary production, our results suggested that adding grazing mobile fishes, led to a more consistent and homogenous supply of algal resources. Because mussels rarely occur in the absence of fish, considering their combined influence on ecosystem dynamics is likely to be important.

Thermal sensitivity modulates temporal patterns of ecosystem functioning by freshwater mussels

“…The distance it took for the mussel community to turnover the N and P pools present in the river was low across all temperatures, but was greatest at 20°C due to relatively low discharge and lower background concentrations of N and P in the river. However, despite the strong, seasonal bottom‐up effects mussels have, mussels co‐occur with fish, and recent work suggests that grazing mobile fish leads to a more consistent and homogeneous supply of resources (Vaughn et al, 2022), thus their combined influence on ecosystem function is likely to be important. In addition, our excretion rates were lower in these lab trials than in field trials on the same species (Hopper et al, 2021) and in comparison to previous field measurements on similar species (Atkinson & Vaughn, 2015), suggesting that laboratory rates may underestimate mussel contributions in‐situ.…”

Freshwater mussels promote functional redundancy in sediment microbial communities under different nutrient regimes

“…We conducted the experiment in outdoor, flow‐through mesocosms at the University of Oklahoma Aquatic Research Facility (ARF; 35°10′58.5″ N 97°26′49.8″ W). These mesocosms have been used successfully in other recent experiments with freshwater mussels (Parr et al, 2020; Vaughn et al, 2022). A pilot study in 2019 demonstrated that by the end of 1 week in a mesocosm, natural sediment microbial communities had significantly decreased in alpha diversity but this change subsequently plateaued.…”