Heather L. Hunt

PATCH DYNAMICS OF MUSSELS ON ROCKY SHORES: INTEGRATING PROCESS TO UNDERSTAND PATTERN

Within any particular landscape, patch structure and dynamics likely will differ among habitat types because of environmental differences influencing the intensity and outcome of biological interactions. We compared the dynamics of natural and experimentally constructed mussel patches (Mytilus trossulus and M. edulis) in two intertidal habitats, tidepools and emergent rock, over a time series (5, 10, 15 mo) and among seasons (three successive 5-mo intervals). In tidepools, mussels naturally occurred in small patches (median <25 cm2). In contrast, mussels on emergent rock formed extensive beds with decimeter-scale gaps, but these beds start as small patches following disturbance such as ice scour. For our experimental patches (15 cm2) in both habitats, we assessed the relative importance of physical (wave disturbance) and biological (predation, growth, recruitment, and immigration) processes in determining patch size and structure. Although individual experimental and natural patches varied greatly in size...

Effects of whelk (Nucella lapillus (L.)) predation on mussel (Mytilus trossulus (Gould), M. edulis (L.)) assemblages in tidepools and on emergent rock on a wave-exposed rocky shore in Nova Scotia, Canada

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 (

RECOVERY OF AN INTERTIDAL ASSEMBLAGE FOLLOWING A RARE OCCURRENCE OF SCOURING BY SEA ICE IN NOVA SCOTIA, CANADA

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

Post-settlement alteration of spatial patterns of soft shell clam (Mya arenaria) recruits

The recovery of an intertidal assemblage of macroalgae and sessile macrofauna was monitored following a rare occurrence of scouring by sea ice on an exposed rocky shore near Halifax, Nova Scotia, Canada. The ice-scour denuded the substratum at all intertidal levels, and the objective of this study was to determine whether the assemblage would return to its original structure. Sampling was done along a 10m transect at about yearly intervals from 7 months before to 5 years and 7 months after the ice-scour. Ephemeral algae blanketed the shore soon after the ice-scour, but these were replaced by perennial macroalgae in less than two years. The characteristic zonation of the macroalgae on the shore before the ice-scour, with a successive replacement of Fucus spp. from the high to mid-shore regions and Chondrus crispus low on the shore, was restored in only two years. The cover of macroalgae at mid-intertidal heights (Fucus vesiculosus, F. evanescens) was re-established more rapidly than the cover high (E spiralis) or low (Chondrus crispus) on the shore. The barnacle Semibalanus balanoides covered less than 10% of the shore before and soon after the ice-scour, and except in the year following the ice-scour when barnacles settled in large densities low on the shore, they were restricted to the midand high shore regions. Mussels (Mytilus edulis and/or M. trossulus) covered more than 50% of the shore before the ice-scour, and their rate of recolonisation was inversely related to height on the shore. After about four years, the cover of mussels in the low and mid-shore regions was similar to that before the ice-scour, but even after almost six years the cover of mussels high on the shore remained less than that before the ice-scour. Many of the observations made during the recovery of this assemblage, such as the rate and sequence of replacements of species at particular heights on the shore, paralleled the results of experimental studies done at much smaller spatial scales within similar, established assemblages.

Temporal genetic heterogeneity within a developing mussel (Mytilus trossulus and M-edulis)assemblage

To examine the roles of settlement and early post-settlement processes in patterns of recruitment of the soft shell clamMya arenaria, abundance of juvenileMya at three intertidal sites in Barnstable Harbor, Massachusetts, was monitored over two settlement seasons. Two peaks of settlement occurred in 1998 (July and September) and one peak was recorded in June 1999, indicating that a late season settlement event is not a consistent feature at this site. Abundance of recent settlers (i.e., early recruits, < 1 week past settlement) varied significantly among the tidal flats (sites) that were separated by hundreds of meters to kilometers, but not among plots meters apart. Differences in abundance of settlers likely resulted in these differences in early-recruit abundance among sites. Settlement was greatest at the site with the greatest variability in flow speed. Sites also differed to some extent in their sediment characteristics and macrofaunal assemblages, which may influence larval substrate choice. Between-site differences in abundance ofMya decreased after settlement. The rate of decline of abundance varied among cohorts and sites. Comparison of abundance of recent settlers (up to 145,000 m−2) to that of juveniles > 3-mm shell length at the end of the settlement season (up to 60 m−2) indicated large losses of individuals during the early post-settlement period. This study demonstrates that spatial patterns inMya abundance can change substantially during the early post-settlement period, and that high mortality rates can result in cohorts contributing little to the population size even when rates of settlement are high.

Effects of CO2-driven sediment acidification on infaunal marine bivalves: A synthesis

The genetic structure of mussel recruits on a rocky shore in Nova Scotia was measured after a rare ice-scouring event which completely removed the intertidal community. Genotype frequencies were measured at three enzyme loci, phosphoglucose mutase (PGM), aminopeptidase-I (product of the leucine aminopeptidase (LAP) locus), and mannose phosphate isomerase (MPI), using cellulose acetate gel electrophoresis. The developing mussel assemblage was a mixture of Mytilus trossulus, M. edulis and their hybrids, with a greater proportion of the former species. Mussel settlers were collected for two years to examine whether genetic heterogeneity existed within and between cohorts of settlers. Settlement of mussels of both species began in April or May and continued into January. Temporal genetic heterogeneity was observed among groups of settlers, resulting from both variations in the relative proportions of the two species and from genetic heterogeneity within M. trossulus. Allele frequencies of mussel cohorts were followed from settlement to ten months post-settlement to investigate the possibility of species-specific selection and of genotype-specific selection within M. trossulus. Temporal genetic heterogeneity primarily was attributed to changes in the proportions of each species, indicating that post-settlement processes were species-specific. Early post-settlement changes were inconsistent, but later changes were clearly directional, resulting in decreased proportions of M. edulis in larger mussels in both years. There was some evidence of genotype-specific post-settlement mortality within M. trossulus, particularly during the early post-settlement period. Our results indicate that temporally variable settlement patterns and post-settlement selection interacted to produce temporal genetic heterogeneity in this mussel assemblage.

Use of the Atlantic nut clam (Nucula proxima) and catworm (Nephtys incisa) in a sentinel species approach for monitoring the health of Bay of Fundy estuaries.

While ocean acidification (OA) effects on marine organisms are well documented, impacts of sediment acidification on infaunal organisms are relatively understudied. Here we synthesize CO2-driven sediment acidification effects on infaunal marine bivalves. While sediment carbonate system conditions can already exceed near-future OA projections, sediments can become even more acidic as overlying seawater pH decreases. Evidence suggests that infaunal bivalves experience shell dissolution, more lesions, and increased mortality in more acidic sediments; effects on heavy metal accumulation appear complex and uncertain. Infaunal bivalves can avoid negative functional consequences of sediment acidification by reducing burrowing and increasing dispersal in more acidic sediments, irrespective of species or life stage; elevated temperature may compromise this avoidance behaviour. The combined effects of sediment acidification and other environmental stressors are virtually unknown. While it is evident that sediment acidification can impact infaunal marine bivalves, more research is needed to confidently predict effects under future ocean conditions.

Spatial patterns of richness and abundance of benthic decapod crustaceans and fishes in the North-west Atlantic as measured by cobble-filled bio-collectors

Designing an effective environmental monitoring system for population responses requires knowledge of the biology of appropriate sentinel species and baseline information on the areas physical and chemical characteristics. This study collected information in Saint John Harbor, NB, Canada, for two abundant marine benthic invertebrates, the Atlantic nut clam (Nucula proxima) and the catworm (Nephtys incisa) to characterize their seasonal and spatial variability, determine the ideal sampling time and methods, and develop baseline data for future studies. We also evaluated whether contamination is impacting invertebrates by comparing sediment metal concentrations to responses of benthic infauna. Metals were generally below sediment quality guidelines except for nickel and arsenic. Clam densities were variable between sites but not seasons, whereas catworm densities were not significantly different between sites or seasons. Overall, these species show potential for environmental monitoring, although investigation at more contaminated sites is warranted to assess their sensitivity.

Role of early post-settlement mortality in recruitment of benthic marine invertebrates

ABSTRACT Examination of spatial patterns of organisms in the rocky subtidal zone is logistically challenging, particularly over large geographic distances. In the present study, we describe patterns of richness and abundance of decapod crustaceans and small demersal fishes colonizing cobble-filled bio-collectors in 2008 and 2009 at nearshore sites across an important oceanographic and biogeographic gradient in the North-west Atlantic from Rhode Island, USA to Newfoundland, Canada. At least 17 decapod and 24 fish genera were caught, including cryptic fish taxa not readily sampled with other gear. Species richness and abundance of decapods and fishes in collectors at shallow sites (5–10 m) was similar among nearby sites and was greatest in the southernmost region, but did not follow a simple latitudinal cline. The lack of clear latitudinal patterns is likely to be a result of the complex geography of summer temperatures along this coast, with the lowest temperatures at the mouth of the Bay of Fundy, the centre of the study domain. In 2008, the abundance of crabs was positively related to temperature while that of shrimp was negatively related. There was also a significant interaction between depth and study area, with greater similarity among areas at greater depths (up to 76 m), and greater similarity among depths in areas with little temperature stratification. Our results indicate that bio-collectors are a powerful tool for sampling decapod crustaceans and small demersal fishes in nearshore rocky habitats. Given the anthropogenic impacts in coastal habitats, this information is likely to prove useful for detecting changes in rocky subtidal habitats.