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Although ocean acidification will impact marine organisms in the future, few studies have addressed the effects of sedimentary porewater acidification on benthic invertebrates. This study suggests that burrowing behavior and post-settlement dispersal of juvenile bivalves are altered by porewater acidification under present day conditions. We tested the efficacy of a novel method of stabilizing porewater pH using sediment underlain with food grade gelatin in both the lab and field, and then employed this method to test if porewater acidification could alter post-settlement clam dispersal under natural conditions. In the field, clams were exposed to a gradient of porewater acidification in manipulated (CO2 added) sediments for 24 h to determine if acidification could alter dispersal patterns of juvenile clams under natural flow conditions; juvenile clam dispersal in the presence of different buffer types was also tested. In addition, juvenile clams were placed on unmanipulated, field-collected sediment cores in the lab which varied naturally with respect to acidification to test burrowing behavior in response to natural porewater pH. Gelatin stabilized porewater pH for 24–48 h and its presence did not influence clam burrowing behavior. In the field, a significant negative relationship between the percent of clams dispersed and acidification was observed, while stabilizing porewater pH significantly decreased clam dispersal. In the lab, there was a significant positive relationship between the percent of clams burrowed and porewater acidification. This study suggests that porewater acidification has the capacity to alter the burrowing behavior and dispersal patterns of juvenile bivalves under natural conditions.

In soft-bottom habitats, post-settlement dispersal of infaunal organisms can be important in population connectivity and turnover. However, this phase of dispersal is often overlooked and understanding of how spatial and temporal variability in biophysical processes drives dispersal in the field is lacking. To quantify spatial and temporal variability in post-settlement dispersal of bivalves, we conducted a study at three sites in Whangarei Harbour, New Zealand. At each site on two separate occasions, we stained the sediment and infauna in 2 m diameter plots with fluorescein to measure the post-settlement dispersal of the clam, Austrovenus stutchburyi (Gray). Dispersal was quantified by monitoring changes in the abundance of non-fluorescent individuals within the plots and fluorescent individuals outside the plots. Our experiments show that some A. stutchburyi were able to disperse at least 50 cm into the plots within 1 tidal cycle (12 h). Dispersal was size dependent, with adult A. stutchburyi dispersing less than juveniles. Post-settlement dispersal varied both spatially and temporally, as indicated by both the change in abundance of non-fluorescent individuals in the plots through time, and the turnover rate within the plots. Dispersal was greatest at the most wave exposed site, and during a storm event. By replicating an experiment in both space and time, our results demonstrate the variability in bivalve dispersal in the field setting. Through the use of fluorescein staining in situ, this study is one of the few that provides estimates of population turnover due to post-settlement dispersal in undisturbed communities.

Although ocean acidification is expected to reduce carbonate saturation and yield negative impacts on open-ocean calcifying organisms in the near future, acidification in coastal ecosystems may already be affecting these organisms. Few studies have addressed the effects of sedimentary saturation state on benthic invertebrates. Here, we investigate whether sedimentary aragonite saturation (Ωaragonite) and proton concentration ([H+]) affect burrowing and dispersal rates of juvenile soft-shell clams (Mya arenaria) in a laboratory flume experiment. Two size classes of juvenile clams (0.5–1.5 mm and 1.51–2.5 mm) were subjected to a range of sediment Ωaragonite and [H+] conditions within the range of typical estuarine sediments (Ωaragonite 0.21–1.87; pH 6.8–7.8; [H+] 1.58 × 10− 8–1.51 × 10- 7) by the addition of varying amounts of CO2, while overlying water pH was kept constant ~ 7.8 (Ωaragonite ~ 1.97). There was a significant positive relationship between the percent of juvenile clams burrowed in still water and Ωaragonite and a significant negative relationship between burrowing and [H+]. Clams were subsequently exposed to one of two different flow conditions (flume; 11 cm s− 1 and 23 cm s− 1) and there was a significant negative relationship between Ωaragonite and dispersal, regardless of clam size class and flow speed. No apparent relationship was evident between dispersal and [H+]. The results of this study suggest that sediment acidification may play an important role in soft-shell clam recruitment and dispersal. When assessing the impacts of open-ocean and coastal acidification on infaunal organisms, future studies should address the effects of sediment acidification to adequately understand how calcifying organisms may be affected by shifting pH conditions.

Recruitment patterns of marine bivalves are influenced by pre-settlement, settlement and post-settlement factors. Settlement patterns of the soft shell clam, Mya arenaria, were recorded at multiple tidal heights at Mill Cove, New Brunswick. It was found that the initial settlement of M. arenaria occurred at all locations on the shore and was significantly related to adult abundance and sediment grain size. At all transects there was a decrease in abundance of M. arenaria within the first month after settlement, but not all transects showed a similar decrease. While predation likely contributed to the decrease in abundance at all locations on the shore, post-settlement dispersal was an important factor in determining changes in distribution of the juvenile bivalves on the shore. By utilizing bedload traps, spatial variation in the movement of both sediment and M. arenaria was quantified. Transplants of stained clams quantified immigration and emigration rates and showed there was emigration away from the low transects, but no net movement at the high transects on the shore. Dispersal rates of recently settled M. arenaria are generally linked to sediment movement. At Mill Cove, tracking of sediment indicated that the movement of sediment increased from 1–2 m day− 1 in the high intertidal to 5–8 m day− 1 in the low intertidal, and all movement was in a shoreward direction. This study demonstrates that post-settlement dispersal can greatly affect the distribution of juvenile bivalves on an intertidal shore.

Although it is recognized that many species of benthic invertebrates continue to disperse after settlement, particularly in soft-bottom habitats, the scale over which movements of juveniles occur is not well known. This study combined laboratory flume experiments assessing the effects of clam size, species, and water velocity on rates and distances of dispersal of three species of juvenile bivalves with field measurements of loss rates and distances of dispersal of transplanted bivalves in the Navesink River estuary in New Jersey, USA. Dispersal distances measured in the laboratory ranged from an average of 1.6 to 40 cm h− 1 depending on clam size, species, and flow speed. Distances and likelihood of dispersal were generally greater for Mya arenaria than for Mercenaria mercenaria or Gemma gemma, although differences between species were not consistent. As predicted, smaller (1.3 mm) M. arenaria tended to disperse more than larger (3.7 mm) ones, although no significant differences were detected between two sizes (1.8 and 3.4 mm) of M. mercenaria. The similarity of the erosion thresholds of dead clams across sizes and species suggests that burrowing behaviour plays an important role in determining variation in dispersal due to clam size and species. In the field, densities of clams (M.arenaria and M.mercenaria) were reduced to half of that in controls after 3.5–5 h, indicating high levels of dispersal and/or mortality. Some individuals were recovered up to 50 cm away from their initial locations. Overall, our results suggest that dispersal distances of these three species due to bedload transport are likely to be on the order of centimeters per hour. Although these dispersal distances are small, such movements are likely to occur frequently due to tidal currents and, consequently, may have profound impacts on patterns of abundance and distribution.

Erosion and transport of juvenile benthic invertebrates, including bivalves, have the potential to alter patterns of distribution and abundance during the early post-settlement period. However, the factors influencing rates of postlarval dispersal are not well understood. Both hydrodynamics and behaviour (e.g. burrowing) are likely to play a role in determining patterns of transport of juvenile bivalves. To determine the relationship between sediment transport and bivalve dispersal, experiments were conducted in a racetrack flume to examine the effect of grain size, flow, and clam size on rates of erosion of two species of juvenile clams (Mya arenaria and Mercenaria mercenaria). Results of the experiments were compared to predictions of erosion thresholds based on the physical characteristics of the sediment and clams. Erosion of Mercenaria was greater than Mya, the opposite of predictions based on Mercenaria's greater density, indicating the importance of burrowing behaviour. In most cases, erosion also was greater in the finer sand, in contrast to the predicted similarity of erosion thresholds of the two sediments. However, clam erosion did increase with increasing shear velocity and decrease with clam size, as expected. The results of this study indicate that both hydrodynamics and behaviour play roles in the transport of these two species of juvenile bivalves and that their vulnerability to passive erosion cannot be predicted solely from knowledge of sediment transport.

The whelk Nucella lapillus is the most abundant predator of intertidal mussels (Mytilus trossulus and M. edulis) on rocky shores along the Atlantic coast of Nova Scotia, Canada. Environmental differences among intertidal habitats, such as tidepools and emergent rock, may influence the intensity of predation and its effect on community structure. We manipulated densities of both recruits ( , 5 mm shell length, SL) and post-recruits ( $ 5m m SL) ofN. lapillus in tidepools and on emergent rock to examine the effects of whelk predation on mussel assemblages on a rocky shore near Halifax, Nova Scotia. Mussels . 10 mm SL were more abundant in plots where the density of whelk post-recruits was reduced than in control plots where their density was not manipulated. Percentage cover of mussels remained stable where the density of post-recruits was reduced but declined in control plots, more so on emergent rock than in tidepools. This between-habitat difference probably reflects differences in the density of whelk post-recruits since feeding rates of whelks enclosed in cages did not differ significantly between tidepools and emergent rock. Predation by whelk post-recruits could not fully account for the reduction in mussel cover and abundance on emergent rock or in tidepools. This discrepancy is probably due to dislodgment by wave action of mussels killed by whelks, as well as the live mussels surrounding the empty shells. We could not detect an effect of recently recruited whelks on mussel cover or size distribution. Laboratory experiments indicated that the size of Nucella lapillus could be predicted from the diameter of the drill hole they create when feeding on a mussel. In the laboratory, feeding rate was linearly related to body size for recruits but not for post-recruits. Mean size of mussels consumed increased with increasing whelk size for both recruits and post-recruits. In the field, the size distribution of shells drilled by post-recruits differed from that of live mussels, but the distribution of shells drilled by recruits was generally similar to that of live mussels. Analysis of field-collected