Rates of post-larval bedload dispersal in a non-tidal soft-sediment system

Abstract

Quantifying rates of dispersal and understanding patterns of colonization are key for predicting disturbance-recovery dynamics. For soft-sediment benthic communities recruitment is not restricted to one single event, but can be highly variable on several spatial and temporal scales. To investigate the temporal persistence of post-larval bedload transport in a non-tidal system, field experiments were conducted at 4 sites (5 m depth) across a wind-wave exposure gradient over 2 summers. Results indicate that bedload dispersal is temporally variable and dependent on an interaction between species-specific characteristics (including seasonal peaks in reproduction), site-specific hydrographic conditions, grain size characteristics and transport of sediments and drift algae. Dispersal of more passive taxa (Ostracoda, juvenile gastropods, Macoma balthica) was found to be associated with higher rates of sediment and algal transport, while more active species (Hydrobia ulvae and Potamopyrgus antipodarum) dispersed relatively more at sheltered sites. At higher rates of sediment transport, there was an increase in the proportion of larger M. balthica individuals dispersing, in contrast to Hydrobiidae, which had an increased proportion of smaller individuals. Our study suggests that frequent post-larval dispersal plays a central role in the population dynamics of many benthic invertebrates, in non-tidal systems. While the distribution of species is heavily influenced by peaked larval recruitment over large spatial scales, the net result of continuous small-scale dispersal events is clearly important and may contribute to the resilience of benthic communities.

Does stability in local community composition depend on temporal variation in rates of dispersal and connectivity?

“…This confirms that overall dispersal is temporally highly variable (not a constant), is species-specific and more even between taxa at times of increased physical connectivity in the region (Table 1, Matias et al, 2012). Rates of dispersal of benthic invertebrates in the bedload (Be), for example, are closely related to the amount as well as type of energy reaching the bottom in combination with site-specific conditions (Valanko et al, 2010b). At times when waves (oscillatory flow component) dominate over an area the potential for energy reaching the bottom (shear stress, U ⁎ ) and exceeding the threshold for erosion of sediment (U ⁎crit ) is non-linearly enhanced (Le Hir et al, 2007).…”

“…Dispersal of soft-sediment benthic invertebrates occurs after initial settlement as juveniles and adults both passively, associated with sediment bedload transport (Commito et al, 1995;Lundquist et al, 2006;Valanko et al, 2010b), and actively (Armonies, 1994;Cummings et al, 1993;Valanko et al, 2010b). Benthic invertebrates may also exhibit relatively fast turnover rates at the local scale in individuals (emigration = immigration, Norkko et al, 2001).…”

“…The local assemblage was analyzed in two complementary ways, first by identifying taxa to the lowest practical/possible level, and secondly by further dividing taxa that exhibited large size differences into size categories. The size-split divisions were made to investigate if temporal stability in the local assemblage, as well as propensity to disperse depends on the size of individuals under consideration Norkko et al, 2001;Valanko et al, 2010b).…”

Effects of Reduced pH on Macoma balthica Larvae from a System with Naturally Fluctuating pH-Dynamics

“…Slowly growing larvae face a risk of delayed settling and higher predation risk in response to the longer time they are required to spend in the plankton. The population dynamics of a bivalve species is largely dependent on successful settlement and recruitment of the post-larvae [48], and dispersal of larval and post-larval stages from adjacent areas aid in maintaining a healthy benthic community [53], [54]. The Baltic Sea is severely eutrophied and experiences large-scale hypoxia, the extent of which varies seasonally, with large areas of the sea floor being permanently hypoxic [28].…”

Thin terrestrial sediment deposits on intertidal sandflats: effects on pore-water solutes and juvenile bivalve burial behaviour

“…Such rejection is of interest because it affects the species' dispersal. In soft-sediment systems in particular, dispersal of post-settlement juveniles significantly contributes to local (within estuary) population connectivity and the regulation and organisation of benthic communities (Dayton et al, 1994;Norkko et al, 2001;Valanko et al, 2010). For example, the potential dispersal of M. liliana post-settlement juveniles on intertidal sandflats is in the order of kilometres per tidal cycle (Petuha et al, 2006), which is similar to the realised dispersal distances of many benthic coastal species with a pelagic larval stage (see Shanks (2009) for a discussion).…”