Abstract
Freshwater mussels (Unionida) can strongly affect nutrient cycling in temperate ecosystems but data from the tropics is lacking. We quantified the effects of mussel filtration, excretion and biodeposition on nutrient and photosynthetic pigment concentrations in a tropical eutrophic lake and mesotrophic river, featuring one non-native and two native species, respectively. Changes in nutrient and pigment concentrations were measured over a 3 h period to assess effects on (1) the water column in field enclosures, and (2) water column and benthos combined in controlled laboratory experiments. In field enclosures in both systems, mussel density and biomass were significantly correlated with the magnitude of reduction in sestonic pigment concentrations. In laboratory experiments, presence of mussels led to reduced PO4 and increased TAN concentrations in both systems, lower combined sestonic and deposited pigment concentrations in the river but increases in the same in the lake. We conclude that excretion by mussels probably accelerated bioseston growth in both systems due to N-fertilisation, an effect that may be particularly common in tropical freshwaters, which are frequently N-limited. However, whilst river mussels reduced bioseston concentrations through rapid filtration, higher rates of N-excretion and/or deposition of undigested bioseston by lake mussels apparently resulted in a net increase of pigment concentrations.
Highlights
Freshwater mussels (Bivalvia: Unionida, hereafter mussels) are sedentary, benthic molluscs, that feed primarily through filter-feeding and can occur in high densities in various freshwater habitats globally (Graf & Cummings, 2015)
Lake water concentrations of chl a and total ammoniacal nitrogen (TAN) were significantly higher than in river water, whilst soluble reactive phosphorus (SRP) and NO3-N concentrations were significantly higher in the river than the lake (Table 1)
It is likely that this increase arises from nitrification of the NH4 excreted by S. woodiana, which may have been amplified by aeration during the experiment and explains the lack of correlation between NO3-N and mussel size (Lewis, 2001)
Summary
Freshwater mussels (Bivalvia: Unionida, hereafter mussels) are sedentary, benthic molluscs, that feed primarily through filter-feeding and can occur in high densities in various freshwater habitats globally (Graf & Cummings, 2015). Their filtering activity, which has been assessed to attain between about 0.1–3 l h-1 mussel-1 (Kryger & Riisgard, 1988; Chowdhury et al, 2016; Cyr et al, 2017 and references therein), affects ecosystems at various trophic levels and compartments (Vaughn et al, 2008). Mussel excretion generally increases the N:P ratio, which can alleviate strict N-limitation in streams and can lead to a subsequent change in algal communities (Atkinson et al, 2013)
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