A process-orientated understanding of ecosystems usually starts with an exploratory analysis of empirical relationships among potential drivers and state variables. While relationships among herbivory, algal cover, and coral recruitment, have been explored in the Caribbean, the nature of such relationships in the Pacific appears to be variable or unclear. Here, we examine potential drivers structuring the benthos and herbivorous fish assemblages of outer-shelf reefs in Micronesia (Palau, Guam and Pohnpei). Surveys were stratified by wave exposure and protection from fishing. High biomass of most herbivores was favoured by high wave exposure. High abundance of large-bodied scarids was associated with low turf abundance, high coral cover, and marine reserves. The remaining herbivores were more abundant in reefs with low coral cover, possibly because space and hence food limitation occur in high-coral-cover reefs. Rugosity had no detectable effect on herbivorous fish abundance once differences in exposure and coral cover were accounted for. At identical depths, high wave exposure was associated with greater volumes (cover 9 canopy height) of macroalgae and algal turfs, which most likely resulted from high primary productivity driven by flow. In exposed areas, macroalgal cover declined as the acanthurid biomass increased. The volume of algal turfs was negatively associated with coral cover and herbivore biomass. In turn, high coral cover and herbivore biomass are likely to intensify grazing. The density of juvenile corals was variable where macroalgal cover was low but was confined to lower densities where macroalgal cover was high. High coral cover and density of juvenile corals were favoured in sheltered habitats. While a weak positive relationship was found between scarid biomass and juvenile coral density, we hypothesise that high scarid densities may hinder juvenile density through increased corallivory. New hypotheses emerged that will help clarify the role of acanthurids, wave exposure, and corallivory in driving the recovery of Pacific coral communities.

Hypoxia is increasing in coastal zones worldwide, with acute effects on demersal fish and benthic invertebrate communities in shallow coastal and estuarine habitats. Less studied are the effects of hypoxia on planktonic larvae of open coastal habitats. Climate change projections suggest intensified hypoxia in open coast upwelling systems, such as the northern California Current Upwelling System, where there has been a recent rise of seasonally persistent inner-shelf hypoxia (dissolved oxygen [DO] < 1.43 ml l^-1) and anoxia (DO 0 ml l^-1). We examined survivorship of larval invertebrates exposed to low oxygen conditions in controlled laboratory experiments. Multiple-day hypoxic conditions, resembling DO levels in nearshore Oregon waters, were generated by bubbling seawater with nitrogen gas. Tolerance levels among species varied, from larvae of species with little tolerance to hypoxia (e.g. the shore crab Hemigrapsus oregonensis) to species with high tolerance (e.g. the California mussel Mytilus californianus). These contrasting responses among open-coast intertidal taxa suggest that chronic hypoxia or anoxia may have different effects on recruitment success among species and, consequently, on the structure and species composition of open coast intertidal communities.

We assessed the utilization of inorganic and organic nitrogen compounds of different complexity by primary producers and bacteria in a seagrass ecosystem. Using double-labeled (¹³C and ¹⁵N) substrates, the net transfers from the dissolved nitrogen and carbon pools to phytoplankton, planktonic bacteria, epiphytes, seagrasses (Zostera noltii and Cymodocea nodosa), and a macroalga (Caulerpa prolifera) were quantified in field incubations. Phytoplankton was the largest nitrogen sink, followed by the epiphytic community. In contrast, carbon fixation was dominated by the macrophytes. Although compartment-specific variations existed, NH₄⁺ was generally preferred over NO₃^- and urea. Specific uptake rates of individual amino acids were inversely related to their C:N ratio and their structural complexity (glycine > L-leucine > L-phenylalanine). In addition, biomarker-specific measurements (polar lipid-derived fatty acids and D-alanine) indicated increasing bacterial contributions to carbon uptake with increasing amino acid structural complexity. All primary producers acquired nitrogen from a complex pool of algae-derived dissolved organic matter (DOM), but algae-derived dissolved organic carbon (DOC) was almost exclusively used by the planktonic compartment. In contrast, a similar complex pool of bacteria-derived DOM was not taken up in significant quantities by any of the primary producers. Our results illustrate that dissolved organic nitrogen (DON) can provide bioavailable nitrogen to primary producers and bacteria in seagrass ecosystems on very short time-scales, and DON and DOC dynamics are largely uncoupled and must be investigated as 2 separate interacting pools.

Gravid adults of the common intertidal barnacle Semibalanus balanoides (L.) brood fully developed larvae until individuals perceive some cue from the environment that triggers synchronous larval release. The prevailing hypothesis has been that phytoplankton blooms trigger release because they provide a food source for nauplius larvae. Through observations and field experiments, we tested the hypothesis that turbidity from any source, not just phytoplankton blooms, can trigger release. We documented 5 larval release events at 3 sites in the northeastern USA. Two events coincided with chlorophyll increases, and all 5 coincided with turbidity increases. In experiments, the larval release response was equivalent when adults were exposed to diatoms or inert synthetic beads, and it was significantly higher than under exposure to filtered seawater. We also tested the hypothesis that turbidity can decrease the risk of cannibalism for newly released nauplii. Under experimentally manipulated conditions, adults consumed significantly fewer nauplii in a high-turbidity environment. We suggest that reproduction in this species may have evolved to coincide roughly with the local onset of winter/spring phytoplankton blooms, but the timing of larval release may have been fine-tuned further by cannibalism and predation pressures. The potential for turbid conditions to serve as a refuge for planktonic larvae of other marine organisms merits further investigation.

The co-existence patterns of 2 mytilid species were studied in order to evaluate the performance of the alien mussel Xenostrobus securis and the indigenous Mytilus galloprovincialis through monospecific and mixed co-habiting experiments in the Ría de Vigo (northwest Spain). Mytilus galloprovincialis was found to be the best performer in all tests. Shell growth was ~4-fold greater for the indigenous mussel when deployed without interspecific competition and regardless of density. Differences in shell growth were found to be similar within mixed populations wherein M. galloprovincialis showed greater ability for upward migratory movements on the mixed beds and distanced itself from the mud influenced layer. The alien X. securis, however, was found to inhabit underneath the M. galloprovincialis layer, which resulted in higher mortality rates. Species-specific features such as byssus filaments and lifestyle may account for such different behaviour. M. galloprovincialis secreted thicker and stronger shells and byssus regardless of their density in both monospecific and mixed experiments. This might indicate that the indigenous mussel performs better when confronted with abiotic stress or biotic pressure. The alien X. securis is currently found to establish high-density patches of individuals at the confluence of main rivers that flow into the estuary (highly heterogeneous scenario). Such behaviour favours its settlement far from the native mussel influence area. X. securis also colonises hard substrates that are not-fully occupied by other invertebrates, but its range was found to be limited by the presence of the indigenous mussel.

In organisms that have a life history phase whose dispersal is influenced by abiotic forcing, if individuals of different species are simultaneously exposed to the same forcing, spatially correlated settlement patterns may result. Such correlated recruitment patterns may affect population and community dynamics. The extent to which settlement or recruitment is spatially correlated among species, however, is not well known. We evaluated this phenomenon among 8 common kelp forest fishes at 8 large reefs spread over 30 km of the coast of Santa Catalina Island, California. In addition to testing for correlated recruitment, we also evaluated the influences of predation and habitat quality on spatial patterns of recruitment. Fish and habitat attributes were surveyed along transects 7 times during 2008. Using these repeated surveys, we also estimated the mortality rate of the prey species that settled most consistently (Oxyjulis californica) and evaluated if mortality was related to recruit density, predator density, or habitat attributes. Spatial patterns of recruitment of the 8 study species were seldom correlated. Recruitment of all species was related to one or more attributes of the habitat, with giant kelp abundance being the most widespread predictor of recruitment. Mortality of O. californica recruits was density-dependent and declined with increasing canopy cover of giant kelp, but was unrelated to predator density. Our results indicate that physical forcing of larval delivery did not generate spatially correlated patterns of recruitment in a suite of temperate-reef fishes.

We investigated the filter-feeding behaviour of individual Mytilus galloprovincialis Lmk in the Ría de Vigo (Galicia, northwest Iberian Peninsula) in order to assess changes in water column biogeochemistry due to mussel culture and the effect of seston organic content on mussel physiological rates. The study was carried out during 1 yr under different oceanographic scenarios by comparing data from a station located in a raft area (RaS) and from a reference site (ReS) with no direct effects of rafting culture activities. Results showed differences between stations in water column nutrient contents, mainly in ammonium levels, with a mean annual excess of 40% at RaS due to mussel feeding activities. We also observed a pronounced deficit of particulate matter at RaS, both in terms of chlorophyll a (chla, -33%) and microbial plankton carbon (-34%), reflecting preferential grazing of mussels on living plankton carbon. Our investigation of the seston organic compound revealed that feeding and digestion of M. galloprovincialis are highly dependent on the quality of the organic composition of the available food. Mussel organic ingestion rate and absorption efficiency were more strongly correlated with microbial plankton carbon contents than with particulate organic carbon and chl a, illustrating the importance of considering not only autotrophic but also heterotrophic microbial plankton carbon as high-quality food for mussels in the Ría de Vigo.

For rocky intertidal invertebrates, the transition from pelagic larva to benthic settler represents a critical life-history stage characterised by high mortality. This mortality has been attributed to biotic factors such as predation or individual larval quality, as well as to abiotic factors such as thermal or desiccation stresses. Surprisingly little is known about how temperature varies at very fine spatial scales relevant to newly settled larvae. We used infrared (IR) imagery to determine (1) whether in situ rocky substrates during aerial exposure exhibit repeatable fine-scale (1 mm) temperature variation at the larval scale, and (2) whether the presence of adult conspecifics ameliorates effects of substratum temperature and promotes early growth and survival of settlers. We tracked the settlement and early survival of larvae to determine whether fine-scale variation in temperature influences early life history processes of the intertidal barnacleTesseropora rosea. Larval settlement did not vary with fine-scale variation in rock temperature, but early post-settlement growth and survival were both inversely related to temperature. Furthermore, we found that rock temperatures decreased significantly with increasing proximity to adult T. rosea and that larvae that settled within 15 mm of adults survived better than those that settled within 16 to 30 mm, highlighting positive effects of gregarious settlement. This is partially explained by conspecific adults shading rock and reducing rock temperatures. We present the first use of IR technology to test for variation in rock temperature at a scale relevant to individual larvae, demonstrating that such fine-scale variation in thermal stress impacts the early-life history stages of a benthic marine invertebrate.

This is the first study to compare defenses to predation in closely related species of Mytilus and draws attention to the role of predation in determining their distribution, abundance and co-existence. We experimentally determined whether 2 blue mussel species, Mytilus edulis and M. trossulus, that co-exist within a hybrid zone in Newfoundland, Canada, differ in their predator-induced responses. Both species recognized cues from predators, although the magnitude of the responses to these cues varied with the predator (Asterias rubens or Cancer irroratus) and the traits measured (growth, attachment strength, shell thickness and adductor muscle mass) after 122 d of exposure. Both mussel species showed decreased growth following exposure to sea stars. Both predator species induced an increase in attachment strength in both mussel species (M. edulis > M. trossulus). M. edulis displayed plasticity in partitioning resources among defensive traits in the presence of sea stars (growth in adductor muscle) or crabs (increased shell thickness). These responses were not observed in M. trossulus, which is therefore likely more susceptible to these predators than is M. edulis. The preference of sea stars for M. trossulus supports this conclusion. Differences in predator-inducible defenses increase the likelihood of M. edulis succeeding M. trossulus in mussel aggregations, which in turn explains the often abrupt increase in the proportion of M. edulis among larger mussels. Such predator-induced defenses, and the resultant predator-prey interactions, affect the temporal and spatial distributions of mussel species and their potential to coexist and hybridize.

The success of marine spatial management and, in particular, the zonation of marine protected areas (MPAs), largely depends on the good understanding of species’ distribution and habitat preferences. Yet, detailed knowledge of fish abundance is often reduced to a few sampled locations and a reliable prediction of this information across broader geographical areas is of major relevance. Generalised additive models (GAMs) were used to describe species−environment relationships and identify environmental parameters that determine the abundance or presence−absence of 11 reef fishes with contrasting life histories in shallow habitats of the Azores islands, Northeast Atlantic. Predictive models were mapped and visualised in a geographic information system (GIS) and areas with potential single or multi-species habitat hotspots were identified. Schooling, pelagic species typically required presence−absence models, whereas  abundance models performed well for benthic species. Depth and distance to sediment significantly described the distribution for nearly all species, whereas the influence of exposure to swell or currents and slope of the seafloor depended on their trophic ecology. Potential presence of single species was widespread across the study area but much reduced for multiple species. There were no habitats shared by high abundances of all species in a given trophic group, and areas shared by minimal abundances were smaller than expected. Potential habitat hotspots should be considered as priority sites for conservation, but were only partially included in the existing MPA network. These findings highlight the potential of this methodology to support scientifically sound conservation planning, including but not restricted to fragmented and constrained habitats, such as those of oceanic archipelagos