Mussels partition resources from natural waters under flowing conditions.

The feeding of three co-occurring freshwater mussel species (Ptychobranchus fasciolaris, Quadrula quadrula and Potamilus alatus) of differing conservation status on river water from their collection site was examined under static and ecologically relevant flow conditions using three characteristics of the suspended river seston from a turbid river (Sydenham River, Ontario): (1) Chlorophyll a fluorescence; (2) size of fluorescent particles determined under flow cytometry; and (3) individual algal taxa identified under flow cytometry. Differences in the clearance rate (CR; water volume cleared of material per mussel and time) based on changes in chlorophyll a concentration were observed among species (P. alatus > Q. quadrula). Mussels had higher CR under flowing conditions and higher CR were observed on four of the nine algal taxa under flowing conditions. Feeding electivity analysis indicated that all mussel species selected for larger particles (28-35 µm size fraction) while rejecting smaller ones (12-19 µm). Whereas mussels did not appear to partition resources by size, mussel species exhibited resource partitioning of algal taxa (i.e., selectively feeding on different algal species; under flowing conditions: small centric diatoms were preferred by P. fasciolaris and Q. quadrula and avoided by P. alatus; Cryptomonas were preferred by P. fasciolaris and avoided by Q. quadrula and P. alatus; Chloromonas were preferred by P. alatus and avoided by P. fasciolaris). This study provides a novel mechanism, hydrodynamically mediated resource partitioning, in which the partitioning of resources occurs from within a flowing fluid rather than through the spatial or temporal partitioning of resources by organisms exploiting different microhabitats, to explain the existence of high mussel species richness within the same river reach (i.e., 24 species in this study). Unfortunately the species specific relationships noted above are likely vulnerable to climate change and changes in land use practice due to agriculture and urbanization.

Adding ecology to particle capture models: numerical simulations of capture on a moving cylinder in crossflow.

The particle capture efficiency, η, of systems that remove suspended particles from ambient flow (e.g. suspension feeding, abiotic pollination) has been studied using static collectors in steady flows. Particle deposition on collectors moving due to fluid flow remains largely unknown, despite its ecological relevance. We used numerical modeling to simulate particle deposition on a 2D circular cylinder subject to flow-induced oscillation in a cross flow. Using parameter values relevant to wind pollination and other natural biological systems, we examined the influence of the direction, amplitude and frequency of the oscillation, the Stokes number (Stk=0.01-5, characterizing particle behavior), as well as the Reynolds number (Re=662 and 3309, characterizing flow regime) in steady and unsteady flow, on η. The numerical model was validated with empirical results for parts of the parameter space. Particle capture occurred via "inertial impaction", "direct interception" and "leeward deposition", as well as via a new mechanism, "collector chasing" for moving collectors. The η of an oscillating cylinder varied significantly relative to a static cylinder, depending on the parameters used, and on the magnitude of a numerical error that caused loss of particles. This variance of η was due to a change in relative momentum between the particle and the moving collector, which depends on Re, Stk and the oscillation parameters. Collector oscillation transverse to oncoming flow direction strongly increased η, whereas collector motion parallel to flow had little effect on capture efficiency. The oscillation also changed leeward capture significantly in some cases. For most conditions, however, leeward deposition was small. Results suggest that collector motion could have significant influence on the particle capture efficiency of natural systems, which indicates the need to incorporate these ecologically more relevant findings into current models. Empirical studies, however, are still necessary to validate these results and provide reliable data.

Diffusivity in a marine macrophyte canopy: implications for submarine pollination and dispersal.

The dispersion and capture of differently shaped particles within a Zostera marina L. (eelgrass; Zosteraceae) bed were examined to understand submarine pollination and other dispersals. During periods of moderate flow in the canopy, the capture rate of "spherical" (the shape of ancestral pollen) and "filamentous" (the shape of eelgrass pollen) particles was greater for particles released at the top of the canopy (3.07 and 4.53% × 10(-5) cm(-2) of collector; i.e., percentage of particles captured normalized to collector area) and greater for filamentous than for spherical particles (4.51% × 10(-5) cm(-2) vs. 2.01% × 10(-5) cm(-2)). Estimates of the horizontal P (Joseph-Sendner diffusion velocity) and the vertical diffusivity (Gaussian K) of filamentous particles were small (P ≈ 4 × 10(-4) m/s; K ≈ 10(-4) m(2)/s) compared to theoretical values that do not consider plant canopies. These findings support the concept that eelgrass canopies modify the fluid dynamics (i.e., reduced turbulent mixing) within their canopies. These results indicate that 1000-10 000 Z. marina pollen are required to pollinate a single flower. Similarly, it was estimated that under some conditions, the probability of particle impaction on eelgrass vegetation approaches certainty. These results provide insight into the evolution of filamentous pollen and submarine pollination, as well as dispersal and other mass transport phenomena within macrophyte canopies.