Surf-zone hydrodynamics alter phytoplankton subsidies affecting reproductive output and growth of tidal filter feeders.

Surf zones, classified from reflective to dissipative, separate the ocean from shore and subsidies from the coastal ocean must pass through surf zones to reach the shore. We have observed that variations in phytoplankton concentrations in the water over the intertidal zone varied with surf-zone hydrodynamics and we hypothesized that this variation would alter growth rates, population structure, and reproductive output of Mytilus californianus and Balanus glandula. From May 2016 to April 2017, along 7 km of Cape Arago, Oregon, USA surf-zone phytoplankton concentrations were determined weekly at nine sites with varying surf-zone hydrodynamics as indicated by surf-zone widths. Throughout the year, concentrations of phytoplankton in wider, more dissipative surf zones were, on average, 16× higher than in narrow, more reflective surf zones. Similar to previous observations, surf-zone width explained >90% of the variability in phytoplankton concentrations in the surf-zone. On average, ~83% of B. glandula had egg lamellae at more dissipative shores compared to only 8% at more reflective. An index of potential reproductive output by barnacle populations as measured by ash free dry mass (AFDM) of egg lamellae was ~243× larger at more dissipative than at reflective shores and surf-zone phytoplankton concentrations and surf-zone width explained ~96 and 92% of the variability in this index. On average, density of M. californianus was ~2× higher at more reflective shores, but 60% of these individuals were smaller and non-reproductive compared to only 24% at the more dissipative sites. The gonad tissue mass/m2 of medium sized mussels was ~5× greater at more dissipative than reflective shores. Surf-zone phytoplankton concentrations and surf-zone width explained ~80% and 65% of the variability in individual mussel gonad mass and ~69% and 56% of the variability in mussel population gonad mass, respectively. M. californianus were out-planted to assess growth rates and, after 5 months, average body mass was ~3× greater at more dissipative than reflective shores. Surf-zone phytoplankton concentrations and width explained ~85% and 92% of the mass increase, respectively. Phytoplankton subsidies varied with surf-zone hydrodynamics altering the growth and reproductive output of two ecologically important intertidal filter feeders.

Surfzone hydrodynamics as a key determinant of spatial variation in rocky intertidal communities.

Larvae of intertidal species develop at sea and must return to adult habitats to replenish populations. Similarly, nutrients, detritus and plankton provide important subsidies spurring growth and reproduction of macroalgae and filter-feeding invertebrates that form the foundation of intertidal communities. Together, these factors determine the density and intensity of interactions among community members. We hypothesized that spatial variation in surfzone hydrodynamics affects the delivery of plankton subsidies. We compared entire zooplankton communities inside and outside the surf zone daily while monitoring physical conditions for one month each at two shores with different surfzone characteristics. Opposite cross-shore distributions of larvae and other zooplankters occurred at the two sites: zooplankton was much more abundant inside the mildly sloping dissipative surf zone (DSZ) with rip currents and was more abundant outside the steep reflective surf zone (RSZ). Biophysical numerical simulations demonstrated that zooplankters were concentrated in rip channels of the DSZ and were mostly unable to enter the RSZ, indicating the hydrodynamic processes behind the observed spatial variation of zooplankters in the surf zone. Differences in the concentration of larvae and other zooplankters between the inner shelf and surf zone may be an underappreciated, key determinant of spatial variation in inshore communities.

Does fish larval dispersal differ between high and low latitudes?

Several factors lead to expectations that the scale of larval dispersal and population connectivity of marine animals differs with latitude. We examine this expectation for demersal shorefishes, including relevant mechanisms, assumptions and evidence. We explore latitudinal differences in (i) biological (e.g. species composition, spawning mode, pelagic larval duration, PLD), (ii) physical (e.g. water movement, habitat fragmentation), and (iii) biophysical factors (primarily temperature, which could strongly affect development, swimming ability or feeding). Latitudinal differences exist in taxonomic composition, habitat fragmentation, temperature and larval swimming, and each difference could influence larval dispersal. Nevertheless, clear evidence for latitudinal differences in larval dispersal at the level of broad faunas is lacking. For example, PLD is strongly influenced by taxon, habitat and geographical region, but no independent latitudinal trend is present in published PLD values. Any trends in larval dispersal may be obscured by a lack of appropriate information, or use of 'off the shelf' information that is biased with regard to the species assemblages in areas of concern. Biases may also be introduced from latitudinal differences in taxa or spawning modes as well as limited latitudinal sampling. We suggest research to make progress on the question of latitudinal trends in larval dispersal.

Juvenile Dungeness crabs (Metacarcinus magister) selectively integrate and modify the fatty acids of their experimental diets.

Dungeness crabs (Metacarcinus magister) are ecologically and economically important in the coastal Northeast Pacific, yet relatively little is currently known about their feeding behaviour in the wild or their natural diet. Trophic biomarkers, such as fatty acids (FA), can be used to reveal trophic interactions. We used two feeding experiments to assess differences in FA composition of juvenile crabs fed different known foods to evaluate how they modify and integrate dietary FA into their own tissues and determine whether crab FA reflect diet changes over a six-week period. These experimental results were then compared with the FA signatures of wild caught juvenile crab with undetermined diets. We found that juvenile Dungeness crabs fed different foods assimilated dietary FA into their tissues and were distinct in their FA signatures when analysed with multivariate statistics. Experimentally fed juvenile crabs contained greater proportions of the most abundant long-chain polyunsaturated fatty acids (LCPUFA, >C20) than their foods. Crabs fed foods lacking in LCPUFA, particularly DHA (22:6ω3, docosahexaenoic acid), did not survive or grew slower than crabs fed other foods. This suggests that LCPUFA are physiologically important for this species and indicates biosynthesis of these FA does not occur or is not sufficient to meet their needs. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.

Pelagic larval duration and dispersal distance revisited.

I present dispersal distances for 44 species with data on propagule duration (PD) for 40 of these. Data were combined with those in Shanks et al. (2003; Ecol. Appl. 13: S159-S169), providing information on 67 species. PD and dispersal distance are correlated, but with many exceptions. The distribution of dispersal distances was bimodal. Many species with PDs longer than 1 day dispersed less than 1 km, while others dispersed tens to hundreds of kilometers. Organisms with short dispersal distances were pelagic briefly or remained close to the bottom while pelagic. Null models of passively dispersing propagules adequately predict dispersal distance for organisms with short PDs (<1 day), but overestimate dispersal distances for those with longer PDs. These models predict that propagules are transported tens of kilometers offshore; however, many types remain within the coastal boundary layer where currents are slower and more variable, leading to lower than predicted dispersal. At short PDs, dispersal distances estimated from genetic data are similar to observed. At long PDs, genetic data generally overestimate dispersal distance. This discrepancy is probably due to the effect of rare individuals that disperse long distances, thus smoothing genetic differences between populations. Larval behavior and species' life-history traits can play a critical role in determining dispersal distance.

Planktonic Subsidies to Surf-Zone and Intertidal Communities.

Plankton are transported onshore, providing subsidies of food and new recruits to surf-zone and intertidal communities. The transport of plankton to the surf zone is influenced by wind, wave, and tidal forcing, and whether they enter the surf zone depends on alongshore variation in surf-zone hydrodynamics caused by the interaction of breaking waves with coastal morphology. Areas with gently sloping shores and wide surf zones typically have orders-of-magnitude-higher concentrations of plankton in the surf zone and dense larval settlement in intertidal communities because of the presence of bathymetric rip currents, which are absent in areas with steep shores and narrow surf zones. These striking differences in subsidies have profound consequences; areas with greater subsidies support more productive surf-zone communities and possibly more productive rocky intertidal communities. Recognition of the importance of spatial subsidies for rocky community dynamics has recently advanced ecological theory, and incorporating surf-zone hydrodynamics would be an especially fruitful line of investigation.

Recruitment limitation in Dungeness crab populations is driven by variation in atmospheric forcing.

Ecologists have long debated the relative importance of biotic interactions vs. abiotic forces on the population dynamics of both marine and terrestrial organisms. Investigation of stock size in Dungeness crab (Cancer magister) is a classic example of this debate. We first tested the hypothesis that adult population size was set by larval success. We found that during a five-year sampling period, adult crab population size from Oregon through central California, USA, as measured by the commercial catch, varied directly with the number of terminal-stage larvae (megalopae) returning to Coos Bay, Oregon, four years earlier; adult population size was largely determined (> 90% of the variation) by success during the larval stage. We then tested whether biotic interactions or abiotic forces caused the variation in larval success. Most of the variation (> 90%) in the number of returning megalopae is explained by the timing of the spring transition, a seasonal shift in atmospheric forcing that drives ocean currents along the west coast of the United States. Early spring transitions lead to larger numbers of returning Dungeness megalopae, while in four other crab taxa, species with very different life history characteristics, early-spring transitions lead to lower numbers of returning megalopae. During the past roughly 30 years, the size of the commercial catch of Dungeness crab is significantly and negatively correlated with the date of the spring transition throughout the California Current system. Long-term variation in the date of the spring transition may explain a major crash in the Dungeness crab fishery in central California, which began in the late 1950s. The data suggest that Dungeness crab population size is determined by variation in larval success and that a significant portion of this variation is due to the timing of the spring transition, a large-scale climatic forcer.

Variation in the abundance of Pseudo-nitzschia and domoic acid with surf zone type.

Most harmful algal blooms (HAB) originate away from the shore and, for them to endanger human health, they must be first transported to shore after which they must enter the surf zone where they can be feed upon by filter feeders. The last step in this sequence, entrance into the surf zone, depends on surf zone hydrodynamics. During two 30-day periods, we sampled Pseudo-nitzschia and particulate domoic acid (pDA) in and offshore of a more dissipative surf zone at Sand City, California (2010) and sampled Pseudo-nitzschia in and out of reflective surf zones at a beach and rocky shores at Carmel River State Beach, California (2011). At Sand City, we measured domoic acid in sand crabs, Emerita analoga. In the more dissipative surf zone, concentrations of Pseudo-nitzschia and pDA were an order of magnitude higher in samples from a rip current than in samples collected just seaward of the surf zone and were 1000 times more abundant than in samples from the shoals separating rip currents. Domoic acid was present in all the Emerita samples and varied directly with the concentration of pDA and Pseudo-nitzschia in the rip current. In the more reflective surf zones, Pseudo-nitzschia concentrations were 1-2 orders of magnitude lower than in samples from 125 and 20m from shore. Surf zone hydrodynamics affects the ingress of Pseudo-nitzschia into surf zones and the exposure of intertidal organisms to HABs on the inner shelf.

Adding teeth to wave action: the destructive effects of wave-borne rocks on intertidal organisms.

Observations in rocky intertidal areas demonstrate that breaking waves 'throw' rocks and cobbles and that these missiles can damage and kill organisms. Targets in the intertidal were dented by impacts from wave-borne rocks. New dents/day in these targets was positively correlated with the daily maximum significant wave height and with new patches/day in aggregations of the barnacle Chthamalus fissus. Impact frequency was highest in the upper intertidal and varied dramatically between microhabitats on individual boulders (edges, tops and faces). These patterns were reflected in the microhabitat abundances of 'old' and 'young' barnacles. Comparisons were made of the survivorship and the frequency of shell damage in two populations of the limpet Lottia gigantea living in habitats which differed primarily in the number of moveable rocks (i.e. potential projectiles). The mortality rate and frequency of shell damage were significantly higher in the projectilerich habitat. In addition only in this habitat did the frequency of shell damage covary significantly with seasonal periods of high surf. Investigation of the response of limpet shells to impacts suggests that shell strength varies between species and increases with shell size. Species-specific patterns of non-fatal shell breakage may have evolved to absorb the energy of impacts. In two of the intertidal habitats studied, wave-borne rock damage was chronic and, at least in part, may have governed the faunal makeup of the community by contributing to the physical 'boundaries" of the environment within which the inhabitants must survive.