SUMMARY1. Metacommunity ecology addresses the situation where sets of local communities are connected by the dispersal of a number of potentially interacting species. Aquatic systems (e.g. lentic versus lotic versus marine) differ from each other in connectivity and environmental heterogeneity, suggesting that metacommunity organisation also differs between major aquatic systems. Here, we review findings from observational field studies on metacommunity organisation in aquatic systems. 2. Species sorting (i.e. species are 'filtered' by environmental factors and occur only at environmentally suitable sites) prevails in aquatic systems, particularly in streams and lakes, but the degree to which dispersal limitation interacts with such environmental control varies among different systems and spatial scales. For example, mainstem rivers and marine coastal systems may be strongly affected by 'mass effects' (i.e. where high dispersal rates homogenise communities to some degree at neighbouring localities, irrespective of their abiotic and biotic environmental conditions), whereas isolated lakes and ponds may be structured by dispersal limitation (i.e. some species do not occur at otherwise-suitable localities simply because sites with potential colonists are too far away). Flow directionality in running waters also differs from water movements in other systems, and this difference may also have effects on the role of dispersal in different aquatic systems. 3. Dispersal limitation typically increases with increasing spatial distance between sites, mass effects potentially increase in importance with decreasing distance between sites, and the dispersal ability of organisms may determine the spatial extents at which species sorting and dispersal processes are most important. 4. A better understanding of the relative roles of species sorting, mass effects and dispersal limitation in affecting aquatic metacommunities requires the following: (i) characterising dispersal rates more directly or adopting better proxies than have been used previously; (ii) considering the nature of aquatic networks; (iii) combining correlative and experimental approaches; (iv) exploring temporal aspects of metacommunity organisation and (v) applying past approaches and statistical methods innovatively for increasing our understanding of metacommunity organisation.

Size is a fundamental organismal trait and an important driver of ecosystem functions. Although large individuals may dominate some functions and provide important habitat structuring effects, intra-specific body size effects are rarely investigated in the context of BEF relationships. We used an in situ density manipulation experiment to explore the contribution of large, deep-burrowing bivalves to oxygen and nutrient fluxes across the sediment-water interface. By manipulating bivalve size structure through the removal of large individuals, we held species identity constant, but altered the trait characteristics of the community. The number of large bivalves was the best predictor of ecosystem functioning. Our results highlight that (a) accounting for body size provides important insights into the mechanisms underpinning biodiversity effects on ecosystem function, and (b) if local disturbances are recurrent, preventing individuals from reaching large sizes, the contribution of large adults may be lost, with largely unknown implications for ecosystem functionality.T here is clear evidence that losses in biodiversity reduce the efficiency of ecosystem functions, including productivity and nutrient cycling 1 , but the actual mechanisms that underpin the positive biodiversityecosystem function (BEF) relationships remain an area of intense research 2,3 . A few species (with unique traits) may in fact dominate certain ecosystem processes [4][5][6] , and recent meta-analyses have indeed shown that species identity effects may be as important as richness effects per se 7 . Improving the mechanistic understanding of BEF relationships and allow prediction of the magnitude of change in ecosystem function following the loss of particular traits 2,8 thus depends on the nature of the ecosystem function(s) of interest and trait composition not only across the resident community but also within species. Trait-based approaches have, however, largely focused on assigning traits to species rather than individuals 9 . Hence, while interspecific differences in trait composition and the subsequent functional contribution to ecosystem processes have been acknowledged, differences in intra-specific trait characteristics are rarely addressed 10 (but see e.g. [11][12][13] ).Body mass is a fundamental organism trait that affects metabolic rate, energy demand and uptake rate 9,14,15 , and is an important characteristic of overall population and community structure through density-mass allometric relationships 14 . Even though high numbers of small individuals may dominate specific ecosystem functions through rapid turnover rates, the fewer large individuals may dominate other functions and provide important habitat structuring effects 16 . Particularly where ecosystem functions relate to the generation of biogenic habitat or organisms mediation of the nature and flux of energy and matter, size matters. This is potentially very important in marine sediments where large organisms can be expected to displace more sediment, pump more water an...