1.       Freshwater bivalves are key ecosystem engineers in both lotic and lentic ecosystems, with strong effects on habitat formation, bioturbation and filtration. Many species are threatened, including the endangered freshwater pearl mussel Margaritifera margaritifera, yet the understanding of how their disappearance might affect wider ecosystem functioning remains limited.  This includes processes such as decomposition of leaf litter, which is potentially influenced by the activities of large mussel aggregations.

We hypothesise that the presence of M. margaritifera increases litter decomposition due to three potential, non-mutually exclusive mechanisms: (1) excretion of inorganic nutrients might stimulate increased microbial activity on leaf litter, (2), the deposition of faeces and pseudofaeces might drive positive “priming effects”, where microbial decomposition of more refractory C is increased by addition of less refractory C (3) both effects might in turn potentially facilitate support of increased abundance/biomass of detritivores, further increasing decomposition.

2.       We investigated this in-situ in a Swedish boreal forest stream by placing thirty 27 L mesh enclosures, containing coarse gravel and fine (for measuring microbially decomposition) as well as coarse (for measuring total decomposition) mesh bags with birch (Betula) leaves, which were stocked with 0, 3, 6, 12 or 24 M. margaritifera mussels. Enclosures were deployed from late autumn (November 2019) to winter (January 2020).

3.       There was no effect of mussel presence on microbially mediated decomposition in the fine mesh bags, and hence no effect for fertilization or priming effects on decomposition.

Total leaf decomposition rates in the coarse mesh bags were lower in all mussel treatments compared to controls, regardless of mussel density. 

4.       This result is unlikely to reflect changes in shredder abundance, given that the numbers of shredding macroinvertebrates across the treatments were similar. Rather, we hypothesise that in the presence of mussels, shredders switched to consuming biodeposits rather than leaf litter. Alternatively, it is possible that the aggregation of mussels reduced near-bed flow, and hence the direct effect of the current on physical fragmentation of the litter.

This study, the first to examine the relationship between unionid mussels and leaf decomposition in a field experiment, confirms the importance of freshwater mussels in indirectly changing leaf decomposition rates, though not in the direction we expected. Our results suggest that the disappearance of freshwater mussels can have indirect effects on key ecosystem processes other than those they directly regulate.

Ecosystem engineers can greatly affect species communities by modifying habitat structures. Freshwater mussels (Unionida) are a threatened group of ecosystem engineers that modify near-benthic flows, increase substrate complexity, capture large amounts of seston and enrich the benthic environment with organic matter. Here, we conducted a manipulative field enclosure experiment to shed light on how an endangered mussel affects benthic communities, primarily by interacting with flow velocity. We deployed thirty 27 L mesh enclosures, with gravel bottoms and different numbers of adult Margaritifera margaritifera mussels (0, 3, 6, 12 or 24 individuals/enclosure) in a Swedish boreal stream over the course of three months from late autumn to winter. Fine particulate organic matter concentration in interstitial water, flow velocity, and the diversity and composition of the macroinvertebrate community that colonised each enclosure were measured. We found that mussels increased species evenness and shifted taxonomic, feeding trait and substrate preference trait community composition. Most of these metrics were affected by the interaction between flow and mussel density, with mussel density often decreasing the effect of flow velocity. Our study supports the idea that the loss of endangered ecosystem engineers can have unintended consequences for species communities.

Dead organic matter such as leaf litter and faeces is an important resource for detrital food webs, and recalcitrant organic matter often decomposes more quickly when mixed with more labile matter in what is known as the priming effect. In freshwaters, this effect can be positive, negative or neutral. Freshwater bivalves have been found to affect leaf litter decomposition in different ways, but the mechanism by which they do so is not known. In this microcosm study, we found that adding mussel biodeposits to leaf litter does not result in a priming effect, but that adding dead cyanobacteria (Arthrospira sp.) does. We placed birch (Betula sp.) leaf litter, either loose or in a bag designed to exclude macroinvertebrates, in 90 microcosms also containing the freshwater isopod Asellus aquaticus. Microcosms were subjected to one of three algal enrichment treatments (none, Arthrospira or Arthrospira mixed with kaolin), and mussel biodeposit treatments (biodeposits produced by mussels fed by the cyanobacteria blends). We measured how decomposition was affected by these different treatments as well as how treatments affected A. aquaticus growth and mortality. We found that A. aquaticus-mediated leaf litter decomposition was more rapid, especially when cyanobacteria, but not biodeposits, were added. Organic matter addition did not affect the microbial decomposition of leaf litter, nor did it affect A. aquaticus growth and mortality. Our findings suggest that benthic enrichment by bivalve biodeposits is not a main mechanism by which bivalves affect the decomposition of leaf litter.

Ecosystem engineers can affect habitat complexity and the ecology of other species. Aggregations of autogenic ecosystem engineers in lotic systems, such as unionid mussels, can increase habitat complexity and change water flow, affecting the diversity and abundance of other benthic species. The effects of unionid mussel aggregations on mobile species such as their own host fishes are less well-studied, but the increase in habitat complexity may change fish behaviour and act as a refugia to water flow. Using stream flumes, we examined how the presence and density (zero/low density/high density) of the freshwater pearl mussel (Margaritifera margaritifera) and water flow (low/high) affected behaviour of three different size classes of young-of the-year brown trout (Salmo trutta) in May, June and August 2023, respectively. We found that the small-sized trout were less likely to swim at high than at low water flow, but increased the likelihood to swim when mussel density increased. The medium-sized trout were also negatively affected by water flow, but this effect decreased when mussels were present and at high density. Large-sized trout were instead less likely to swim when mussel densities were high, and in contrast to the small- and medium-sized trout, positioned themselves in proximity to the mussels. Such spatial matching may increase mussels’ larval infestation of trout, which occurs by this time of the summer. The results thus suggest that not only host fish, but also mussels indirectly, may benefit from mussel presence. Anthropogenic impacts have negatively affected both species, and we suggest that conservation efforts targeting mussels and their host fish should be considered in impacted streams.