Testate amoebae (Protozoa) were studied in spring, summer, and fall from the same microhabitats in a small Sphagnum-dominated peatland in southern Ontario, Canada. A total of 32 sampling stations were established in two wetland plant communities, 19 in an open Ericaceae low-shrub community and 13 in a closed Picea mariana and Larix laricina swamp community. Sphagnum was collected in each station for analysis of testate amoebae and measurement of soil water content parameters and water table depth in May, August, and October 2001. pH and dissolved oxygen of the groundwater under the Sphagnum were measured also. A total of 52 taxa including the rotifer, Habrotrocha angusticollis, were identified. Soil water content and water table variables emerged as the primary factors separating testate amoebae between the open bog/fen community and swamp community. Testate amoebae in the open bog/fen community showed a clear separation between the May sampling period and the August and October sampling periods. Sampling stations in May had much higher water table and were wetter than those in August and October. Conversely, testate amoebae in the swamp community did not show a clear difference between sampling periods. Soil moisture and water tables appear to be more constant in the swamp communities. Biological factors or other microscale environmental factors may need to be considered to explain seasonal changes in testate amoebae. A greater understanding of relationships between testate amoebae and microenvironmental factors is necessary to track seasonality in testate amoebae distributions.

Many broadcast-spawning benthic invertebrates are subject to sperm limitation yet achieve high population densities, as for example dreissenid mussels (Dreissena polymorpha and Dreisssena bugensis) that were introduced into the Laurentian Great Lakes. The question remains whether biological or ecological/physical mechanisms reduce sperm limitation. Gamete dilution/longevity experiments were undertaken to determine whether dreissenid mussels are subject to sperm limitation, and computational fluid dynamic modeling was used to determine the potential influence of bottom roughness on sperm dilution in nature. Results indicated that dreissenid mussels may be sperm limited, but the extent to which sperm dilution affects them is lower than what was reported for other broadcast spawning invertebrates. Importantly, model mussel clusters influenced external fertilization by retaining sperm in downstream eddies but allowing downstream transport from one cluster to another. This, in addition to high sperm potency at low sperm concentrations, may help to explain the success of dreissenid mussels as invasive species.

Lay Abstract Many marine and some freshwater bottom‐dwelling (benthic) invertebrates broadcast their gametes into the water column where fertilization occurs. The relatively slow swimming speed and rapid dilution of sperm by water currents is thought to limit fertilization, even though species that reproduce this way can be extremely successful. We examined how water velocity and bottom roughness affected the fertilization success of zebra and quagga mussels (Dreissena polymorpha and D. bugensis) in a laboratory flow chamber and in Lake Erie. Our results demonstrate that velocity gradients dilute the concentration of sperm to levels that can lead to sperm limitation. We also found that the strength and pattern of the turbulence in the water flow near the bottom had a strong effect on fertilization. Bottom roughness that led to the ejection of fluid away from the bottom—such as what occurs downstream of a patch of mussels—contributed to higher fertilization success than bottom roughness that led to skimming flows or sweeps of flow toward the bed. Bottom roughness needs to be considered in biological and other transport processes occurring near the bottom. Biologically, dreissenid mussels are the first freshwater benthic organisms that have been shown to be sperm limited. Ecologically, the presence of their shells in the flow chamber and on the lakebed in Lake Erie created sufficient roughness to affect fertilization success. In other words, the mussels had changed the physical environment in a way that favored their reproduction. This new observation helps to explain why dreissenid mussels are successful invaders of freshwater ecosystems.

Summary Larval transport and settlement in benthic invertebrates is theorised to be influenced by bottom roughness and the hydrodynamic forces that roughness generates near the bed. This study of freshwater Dreissena spp. bivalves examined the transport and suspension of pediveliger larvae and larval models in a laboratory flow chamber and larval settlement in Lake Erie. Particle image velocimetry (PIV) measurements in the laboratory, and acoustic Doppler velocimetry (ADV) measurements in the field, were used to estimate small‐scale turbulence in the near‐bed environment over differing roughness. Quadrant analysis was used to determine the frequency of turbulent sweeps and ejections, and the extent of roughness flow regime (skimming versus wake interference flow) was noted to understand the determinants of larval transport and settlement. Skimming flow generated above bottoms with high mussel densities had significantly lower suspended transport (i.e. suspension off the bottom in the flow chamber; 3.31 ± 1.14%) and lower larval settlement in the field (1526 ± 80 larvae m−2 per day) compared to low mussel densities (6.63 ± 1.54% and 1853 ± 47 larvae m−2 per day). Conversely, wake interference flow indicated by high frequencies of turbulent sweeps and ejections generated by the roughness due to mussel patches resulted in high suspended transport in the flow chamber (i.e. via ejections) and the highest larval settlement in the field (i.e. via sweeps; 1943 ± 59 larvae m−2 per day). The spatial configuration of mussel roughness influenced the creation and magnitude of skimming versus wake interference flow, which can inhibit or enhance larval settlement, respectively.

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