Back to the future: using historical climate variation to project near‐term shifts in habitat suitable for coast redwood

Studies that model the effect of climate change on terrestrial ecosystems often use climate projections from downscaled global climate models (GCMs). These simulations are generally too coarse to capture patterns of fine‐scale climate variation, such as the sharp coastal energy and moisture gradients associated with wind‐driven upwelling of cold water. Coastal upwelling may limit future increases in coastal temperatures, compromising GCMs’ ability to provide realistic scenarios of future climate in these coastal ecosystems. Taking advantage of naturally occurring variability in the high‐resolution historic climatic record, we developed multiple fine‐scale scenarios of California climate that maintain coherent relationships between regional climate and coastal upwelling. We compared these scenarios against coarse resolution GCM projections at a regional scale to evaluate their temporal equivalency. We used these historically based scenarios to estimate potential suitable habitat for coast redwood (Sequoia sempervirens D. Don) under ‘normal’ combinations of temperature and precipitation, and under anomalous combinations representative of potential future climates. We found that a scenario of warmer temperature with historically normal precipitation is equivalent to climate projected by GCMs for California by 2020–2030 and that under these conditions, climatically suitable habitat for coast redwood significantly contracts at the southern end of its current range. Our results suggest that historical climate data provide a high‐resolution alternative to downscaled GCM outputs for near‐term ecological forecasts. This method may be particularly useful in other regions where local climate is strongly influenced by ocean–atmosphere dynamics that are not represented by coarse‐scale GCMs.     

Distribution, abundance, size and recruitment of the mussel, Mytilus californianus, across a major oceanographic and biogeographic boundary at Point Conception, California, USA

The Point Conception, California, USA region (hereafter PC) is one of the most important biogeographic and oceanographic discontinuities on the US west coast. Here we address how mesoscale oceanographic and environmental variability in the region around PC, CA may influence the distribution, abundance and size of the mussel Mytilus californianus, a competitively dominant species in rocky intertidal assemblages along the northeast Pacific. Strong upwelling and high wave exposure dominate the California coast north of PC, and weak, seasonal upwelling and warmer water temperatures are characteristic of the region south/east of PC. We hypothesized that the gradient in temperature, upwelling, and wave exposure around PC would greatly influence patterns of recruitment and abundance of mussels, potentially underlying large-scale differences in community structure. We evaluated these predictions by surveying intertidal community structure, mussel distribution, size, abundance and recruitment at a range of intertidal sites around PC. We found that intertidal communities north of PC were dominated mainly by macrophytes, while mussels and barnacles were relatively scarce. Intertidal communities south of PC were dominated by mussels and barnacles, with a low abundance of macrophytes. Mussels were larger and mussel beds were more expansive and extended lower in elevation at sites ranging from north to south around PC. At northern sites, high abundances of sea star predators and elevated wave exposure effectively displaced the entire mussel zone upwards. We found no differences in the numbers of mussel recruits to sites around PC, suggesting that spatial patterns of mussel abundance were not driven by differential recruitment. These results suggest that unlike other well-studied systems, supply of benthic larvae does not underly the large-scale gradient in community structure around PC. We suggest that environmental conditions favorable to macroalgal growth north of PC, and conditions favorable to filter-feeder growth south of PC may underly mesoscale patterns of intertidal community structure in this region.     

INVERTEBRATE‐MEDIATED NUTRIENT LOADING INCREASES GROWTH OF AN INTERTIDAL MACROALGA1

Even in nitrogen‐replete ecosystems, microhabitats exist where local‐scale nutrient limitation occurs. For example, coastal waters of the northeastern Pacific Ocean are characterized by high nitrate concentrations associated with upwelling. However, macroalgae living in high‐zone tide pools on adjacent rocky shores are isolated from this upwelled nitrate for extended periods of time, leading to nutrient limitation. When high‐intertidal pools are isolated during low tide, invertebrate‐excreted ammonium accumulates, providing a potential nitrogen source for macroalgae. I quantified the influence of mussels (Mytilus californianus Conrad) on ammonium accumulation rates in tide pools. I then evaluated the effects of ammonium loading by mussels on nitrogen assimilation and growth rates of Odonthalia floccosa (Esp.) Falkenb., a common red algal inhabitant of pools on northeastern Pacific rocky shores. Odonthalia was grown in artificial tide pool mesocosms in the presence and absence of mussels. Mesocosms were subjected to a simulated tidal cycle mimicking emersion and immersion patterns of high‐intertidal pools on the central Oregon coast. In the presence of mussels, ammonium accumulated more quickly in the mesocosms, resulting in increased rates of nitrogen assimilation into algal tissues. These increased nitrogen assimilation rates were primarily associated with higher growth rates. In mesocosms containing mussels, Odonthalia individuals added 41% more biomass than in mesocosms without mussels. This direct positive effect of mussels on macroalgal biomass represents an often overlooked interaction between macroalgae and invertebrates. In nutrient‐limited microhabitats, such as high‐intertidal pools, invertebrate‐excreted ammonium is likely an important local‐scale contributor to macroalgal productivity.     

Spatial patterns of growth in the mussel, Mytilus californianus, across a major oceanographic and biogeographic boundary at Point Conception, California, USA

The Point Conception, California, USA region (hereafter PC) is one of the most important biogeographic and oceanographic discontinuities on the US west coast. Here we address how mesoscale oceanographic variability in the region around PC affects the growth of the competitively dominant species in the rocky intertidal assemblage: the mussel Mytilus californianus. Strong upwelling and high wave exposure dominate the California coast north of PC, and weak, seasonal upwelling and warmer water temperatures are characteristic of the region south/east of PC. We hypothesized that the oceanographic gradients in temperature, upwelling, wave exposure and productivity around PC would exert strong bottom–up influences on growth rates of mussels, potentially underlying large-scale differences in community structure around the PC region. We evaluated these predictions by measuring mussel growth rates across the PC region both in the intertidal and offshore on moorings. Intertidal mussels grew at much higher rates at sites south relative to north of PC and growth rates decreased in a gradient from south to north. The gradient in intertidal mussel growth around PC was uncorrelated with inshore concentrations of chlorophyll-a, and was most strongly correlated with the alongshore gradient in wave exposure and intertidal temperature. Mussels on moorings offshore from the intertidal sites grew at much higher rates than those in the corresponding intertidal areas, and mussel growth rates did not differ significantly among moored locations around PC. The gradient of increasing temperature from north to south among mooring sites was correlated with a decreasing gradient in productivity in the same direction, potentially contributing to equal and opposite effects on mussel growth at offshore moorings. This study suggests that environmental factors such as cold temperatures and high wave exposure contribute to the spatial pattern of decreasing mussel growth rates from south to north around PC, underlying large-scale patterns of community structure in this region.     

Physical effects of habitat‐forming species override latitudinal trends in temperature

Latitudinal and elevational temperature gradients (LTG and ETG) play central roles in biogeographical theory, underpinning predictions of large‐scale patterns in organismal thermal stress, species' ranges and distributional responses to climate change. Yet an enormous fraction of Earth's taxa live exclusively in habitats where foundation species modify temperatures. We examine little‐explored implications of this widespread trend using a classic model system for understanding heat stresses – rocky intertidal shores. Through integrated field measurements and laboratory trials, we demonstrate that thermal buffering by centimetre‐thick mussel and seaweed beds eliminates differences in stress‐inducing high temperatures and associated mortality risk that would otherwise arise over 14° of latitude and ~ 1 m of shore elevation. These results reveal the extent to which physical effects of habitat‐formers can overwhelm broad‐scale thermal trends, suggesting a need to re‐evaluate climate change predictions for many species. Notably, inhabitant populations may exhibit deceptive resilience to warming until refuge‐forming taxa become imperiled.     

The surf zone: a semi-permeable barrier to onshore recruitment of invertebrate larvae?

The supply of larvae to the shore is important for population replenishment and intertidal community dynamics but its variability at most scales is not well understood. We tested the relationship between nearshore mussel larval abundance and intertidal settlement rates over several years at multiple spatiotemporal scales in Oregon and New Zealand. Abundance of competent larvae nearshore and intertidal recruitment rates were simultaneously quantified using collectors mounted at different depths on moorings 50-1100 m from shore, and at adjacent rocky intertidal sites. Total mussel larval abundance and oceanographic conditions were also measured in some locations. At all scales, abundance of nearshore mussel larvae was unrelated to intertidal recruitment rates. In the intertidal, patterns of mussel recruitment were persistent in space, with sites of consistently high or low recruitment. In contrast, nearshore competent larval abundance showed generally similar abundances among sites except for a high, and spatially-inconsistent, variability in Oregon during 1998 only. On moorings, recruitment tended to be greater on midwater collectors than shallower or deeper. Finally, on moorings larval abundance in traps and recruitment on collectors was unrelated. These results suggest that (1) among sites, the size of the nearshore larval pool is relatively uniform while onshore recruitment varies and is unrelated to larval abundance, (2) temporal variability in nearshore larval availability is not strongly expressed onshore, (3) vertical stratification of competent larvae nearshore is strong and may influence transport and recruitment, and (4) within-coast variability in onshore recruitment is strongly driven by processes occurring locally within the surf zone that need to be studied to understand coastal recruitment dynamics.     

Greenhouse gas,upwelling‐favorable winds,and the future of coastal ocean upwelling ecosystems

Coastal ocean upwelling ecosystems generally represent the most productive large marine ecosystems of the world's oceans, in terms of both primary production rates and tonnages of exploitable fish produced. The Peruvian upwelling system, in particular, stands out as a major factor in world fish production. The Pacific trade winds have traditionally been considered to be the primary driving force for the upwelling system off Peru, but are projected to weaken as climate change proceeds. This leads to concern that the upwelling process in the Peru system, to which its productivity is linked, may likewise weaken. However, other mechanisms involving greenhouse‐associated intensification of thermal low‐pressure cells over the coastal landmasses of upwelling regions suggest general intensification of wind‐driven ocean upwelling in coastal upwelling regions of the world's oceans. But although certain empirical results have supported this expectation, it has not been consistently corroborated in climate model simulations, possibly because the scale of the coastal intensification may be small relative to the scales that are appropriately reflected in the standard models. Here we summarize available evidence for the intensification mechanism and present a proxy test that uses variations in water vapor, the dominant natural greenhouse gas, to offer multiple‐realization empirical evidence for action of the proposed mechanism in the real world situation. While many potential consequences to the future of marine ecosystems would codepend on climate change‐related changes in the thermocline and nutricline structures, an important subset, involving potential increased propensities for hypoxia, noxious gas eruptions, toxic red tide blooms, and/or jellyfish outbreaks, may depend more directly on changes in the upwelling‐favorable wind itself. A prospective role of fisheries in either mitigating or reinforcing this particular class of effects is suggested.     

Top-down and bottom-up community regulation in marine rocky intertidal habitats

Strong top-down control by consumers has been demonstrated in rocky intertidal communities around the world. In contrast, the role of bottom-up effects (nutrients and productivity), known to have important influences in terrestrial and particularly freshwater ecosystems, is poorly known in marine hard-bottom communities. Recent studies in South Africa, New England, Oregon and New Zealand suggest that bottom-up processes can have important effects on rocky intertidal community structure. A significant aspect of all of these studies was the incorporation of processes varying on larger spatial scales than previously considered (10’s to 1000’s of km). In all four regions, variation in oceanographic factors (currents, upwelling, nutrients, rates of particle flux) was associated with different magnitudes of algal and/or phytoplankton abundance, availability of particulate food, and rates of recruitment. These processes led to differences in prey abundance and growth, secondary production, consumer growth, and consumer impact on prey resources. Oceanographic conditions therefore may vary on scales that generate ecologically significant variability in populations at the bottom of the food chain, and through upward-flowing food chain effects, lead to variation in top-down trophic effects. I conclude that top-down and bottom-up processes can be important joint determinants of community structure in rocky intertidal habitats, and predict that such effects will occur generally wherever oceanographic ‘discontinuities’ lie adjacent to rocky coastlines. I further argue that increased attention by researchers and of funding agencies to such benthic–pelagic coupling would dramatically enhance our understanding of the dynamics of marine ecosystems.     

Local scale processes drive long‐term change in biodiversity of sandy beach ecosystems

Evaluating impacts to biodiversity requires ecologically informed comparisons over sufficient time spans. The vulnerability of coastal ecosystems to anthropogenic and climate change‐related impacts makes them potentially valuable indicators of biodiversity change. To evaluate multidecadal change in biodiversity, we compared results from intertidal surveys of 13 sandy beaches conducted in the 1970s and 2009–11 along 500 km of coast (California, USA). Using a novel extrapolation approach to adjust species richness for sampling effort allowed us to address data gaps and has promise for application to other data‐limited biodiversity comparisons. Long‐term changes in species richness varied in direction and magnitude among beaches and with human impacts but showed no regional patterns. Observed long‐term changes in richness differed markedly among functional groups of intertidal invertebrates. At the majority (77%) of beaches, changes in richness were most evident for wrack‐associated invertebrates suggesting they have disproportionate vulnerability to impacts. Reduced diversity of this group was consistent with long‐term habitat loss from erosion and sea level rise at one beach. Wrack‐associated species richness declined over time at impacted beaches (beach fill and grooming), despite observed increases in overall intertidal richness. In contrast richness of these taxa increased at more than half (53%) of the beaches including two beaches recovering from decades of off‐road vehicle impacts. Over more than three decades, our results suggest that local scale processes exerted a stronger influence on intertidal biodiversity on beaches than regional processes and highlight the role of human impacts for local spatial scales. Our results illustrate how comparisons of overall biodiversity may mask ecologically important changes and stress the value of evaluating biodiversity change in the context of functional groups. The long‐term loss of wrack‐associated species, a key component of sandy beach ecosystems, documented here represents a significant threat to the biodiversity and function of coastal ecosystems.     

Environmental drivers of mussels flesh yield in a coastal upwelling system

Eastern boundary coastal upwelling ecosystems (EBUEs) are highly sensitive to climate variability, particularly to coastal wind change. Here, we test the response of the flesh yield of blue mussels cultured in the northern boundary of the Iberian–Canary current EBUE to climate-related variables. Significant relationships were found between the annual mean, seasonal build-up and phenology of the mussel flesh yield with meteorological variables such as continental runoff, intensity and direction of coastal winds, and solar radiation. Our analysis shows that better flesh yields occur during years characterised by dry winters, accompanied by early springs and followed by summers dominated by strong northerly winds that produce intense upwelling. Compared with other EBUEs, upwelling has weakened in the study area over the last fifty years, implying an overall decrease in mussel flesh yield. However, future climate scenarios suggest that coastal upwelling will intensify over the 21th century, particularly during the summer months, which would lead to a recovery of mussel flesh yield.     

Community regulation: the relative importance of recruitment and predation intensity of an intertidal community dominant in a seascape context

Predicting the strength and context-dependency of species interactions across multiple scales is a core area in ecology. This is especially challenging in the marine environment, where populations of most predators and prey are generally open, because of their pelagic larval phase, and recruitment of both is highly variable. In this study we use a comparative-experimental approach on small and large spatial scales to test the relationship between predation intensity and prey recruitment and their relative importance in shaping populations of a dominant rocky intertidal space occupier, mussels, in the context of seascape (availability of nearby subtidal reef habitat). Predation intensity on transplanted mussels was tested inside and outside cages and recruitment was measured with standard larval settlement collectors. We found that on intertidal rocky benches with contiguous subtidal reefs in New Zealand, mussel larval recruitment is usually low but predation on recruits by subtidal consumers (fish, crabs) is intense during high tide. On nearby intertidal rocky benches with adjacent sandy subtidal habitats, larval recruitment is usually greater but subtidal predators are typically rare and predation is weaker. Multiple regression analysis showed that predation intensity accounts for most of the variability in the abundance of adult mussels compared to recruitment. This seascape-dependent, predation-recruitment relationship could scale up to explain regional community variability. We argue that community ecology models should include seascape context-dependency and its effects on recruitment and species interactions for better predictions of coastal community dynamics and structure.     

Ecological implications of extreme events: footprints of the 2010 earthquake along the Chilean coast

Deciphering ecological effects of major catastrophic events such as earthquakes, tsunamis, volcanic eruptions, storms and fires, requires rapid interdisciplinary efforts often hampered by a lack of pre-event data. Using results of intertidal surveys conducted shortly before and immediately after Chile's 2010 M(w) 8.8 earthquake along the entire rupture zone (ca. 34-38°S), we provide the first quantification of earthquake and tsunami effects on sandy beach ecosystems. Our study incorporated anthropogenic coastal development as a key design factor. Ecological responses of beach ecosystems were strongly affected by the magnitude of land-level change. Subsidence along the northern rupture segment combined with tsunami-associated disturbance and drowned beaches. In contrast, along the co-seismically uplifted southern rupture, beaches widened and flattened increasing habitat availability. Post-event changes in abundance and distribution of mobile intertidal invertebrates were not uniform, varying with land-level change, tsunami height and coastal development. On beaches where subsidence occurred, intertidal zones and their associated species disappeared. On some beaches, uplift of rocky sub-tidal substrate eliminated low intertidal sand beach habitat for ecologically important species. On others, unexpected interactions of uplift with man-made coastal armouring included restoration of upper and mid-intertidal habitat seaward of armouring followed by rapid colonization of mobile crustaceans typical of these zones formerly excluded by constraints imposed by the armouring structures. Responses of coastal ecosystems to major earthquakes appear to vary strongly with land-level change, the mobility of the biota and shore type. Our results show that interactions of extreme events with human-altered shorelines can produce surprising ecological outcomes, and suggest these complex responses to landscape alteration can leave lasting footprints in coastal ecosystems.     

Strong genetic clines and geographical variation in gene flow in the rocky intertidal barnacle Balanus glandula

A long-standing issue in marine biology is identifying spatial scales at which populations of sessile adults are connected by planktonic offspring. We examined the genetic continuity of the acorn barnacle Balanus glandula, an abundant member of rocky intertidal communities of the northeastern Pacific Ocean, and compared these genetic patterns to the nearshore oceanography described by trajectories of surface drifters. Consistent with its broad dispersal potential, barnacle populations are genetically similar at both mitochondrial (cytochrome oxidase I) and nuclear (elongation factor 1-alpha) loci across broad swaths of the species' range. In central California, however, there is a striking genetic cline across 475 km of coastline between northern and southern populations. These patterns indicate that gene flow within central California is far more restricted spatially than among other populations. Possible reasons for the steep cline include the slow secondary introgression of historically separated populations, a balance between diversifying selection and dispersal, or some mix of both. Geographic trajectories of oceanic drifters closely parallel geographical patterns of gene flow. Drifters placed to the north (Oregon; approximately 44 degrees N) and south (Santa Barbara, California; approximately 34 degrees N) of the cline disperse hundreds of kilometers within 40 days, yet over the long-term their trajectories never overlapped. The lack of communication between waters originating in Oregon and southern California probably helps to maintain strong genetic differentiation between these regions. More broadly, the geographical variation in gene flow implies that focusing on species-level averages of gene flow can mask biologically important variance within species which reflects local environmental conditions and historical events.     

Sea Surface Temperature Influence on Terrestrial Gross Primary Production along the Southern California Current

Some land and ocean processes are related through connections (and synoptic-scale teleconnections) to the atmosphere. Synoptic-scale atmospheric (El Niño/Southern Oscillation [ENSO], Pacific Decadal Oscillation [PDO], and North Atlantic Oscillation [NAO]) decadal cycles are known to influence the global terrestrial carbon cycle. Potentially, smaller scale land-ocean connections influenced by coastal upwelling (changes in sea surface temperature) may be important for local-to-regional water-limited ecosystems where plants may benefit from air moisture transported from the ocean to terrestrial ecosystems. Here we use satellite-derived observations to test potential connections between changes in sea surface temperature (SST) in regions with strong coastal upwelling and terrestrial gross primary production (GPP) across the Baja California Peninsula. This region is characterized by an arid/semiarid climate along the southern California Current. We found that SST was correlated with the fraction of photosynthetic active radiation (fPAR; as a proxy for GPP) with lags ranging from 0 to 5 months. In contrast ENSO was not as strongly related with fPAR as SST in these coastal ecosystems. Our results show the importance of local-scale changes in SST during upwelling events, to explain the variability in GPP in coastal, water-limited ecosystems. The response of GPP to SST was spatially-dependent: colder SST in the northern areas increased GPP (likely by influencing fog formation), while warmer SST at the southern areas was associated to higher GPP (as SST is in phase with precipitation patterns). Interannual trends in fPAR are also spatially variable along the Baja California Peninsula with increasing secular trends in subtropical regions, decreasing trends in the most arid region, and no trend in the semi-arid regions. These findings suggest that studies and ecosystem process based models should consider the lateral influence of local-scale ocean processes that could influence coastal ecosystem productivity.     

Inter-hemispheric comparison of bottom-up effects on community structure: Insights revealed using the comparative-experimental approach

The comparative-experimental approach uses identically designed, replicated experiments at different sites along environmental gradients in order to gain insight into the changing dynamics of communities with changing environmental conditions. Such studies reveal how ecological processes vary in intensity and interact to produce community structure. Early emphases were on the community consequences of shifting top-down impacts, competition and disturbance with environmental stress. Recent advances include the more precise quantification of gradients and thus a better understanding of species responses to the environment, and the revelation that bottom-up forces can vary significantly on within-region scales, with major consequences for the impact of top-down forces and thus community dynamics. Here the use of the method to examine the role of geographic location (coastal ecosystems in different hemispheres) and oceanographic conditions (upwelling vs downwelling) on these bottom-up/top-down linkages is advanced. We show that a bottom-up factor (prey recruitment) and a top-down effect (predation rate) vary consistently with oceanographic conditions within each coastal ecosystem, and also between geographic locations (New Zealand, Oregon). In general, both recruitment and predation rates are higher in Oregon. It is postulated that these differences are common responses to oceanographic variation, and that between-hemisphere differences result from the stronger and more persistent upwelling in the California Current ecosystem.     

Response of a rocky intertidal ecosystem engineer and community dominant to climate change

To evaluate how climate change might impact a competitively dominant ecological engineer, we analysed the growth response of the mussel Mytilus californianus to climate patterns [El Niño-Southern Oscillation, Pacific Decadal Oscillation (PDO)]. Mussels grew faster during warmer climatic events. Growth was initially faster on a more productive cape compared to a less productive cape. Growth rates at the two capes merged in 2002, coincidentally with a several year-long shift from warm to cool PDO conditions. To determine the mechanism underlying this response, we examined growth responses to intertidal sea and air temperatures, phytoplankton, sea level and tide height. Together, water temperature (32%) and food (12.5%) explained 44.5% of the variance in mussel growth; contributions of other factors were not significant. In turn, water temperature and food respond to climate-driven variation in upwelling and other, unknown factors. Understanding responses of ecosystem engineers to climate change will require knowing direct thermal effects and indirect effects of factors altered by temperature change.     

Multiple stressors,nonlinear effects and the implications of climate change impacts on marine coastal ecosystems

Global climate change will undoubtedly be a pressure on coastal marine ecosystems, affecting not only species distributions and physiology but also ecosystem functioning. In the coastal zone, the environmental variables that may drive ecological responses to climate change include temperature, wave energy, upwelling events and freshwater inputs, and all act and interact at a variety of spatial and temporal scales. To date, we have a poor understanding of how climate‐related environmental changes may affect coastal marine ecosystems or which environmental variables are likely to produce priority effects. Here we use time series data (17 years) of coastal benthic macrofauna to investigate responses to a range of climate‐influenced variables including sea‐surface temperature, southern oscillation indices (SOI, Z4), wind‐wave exposure, freshwater inputs and rainfall. We investigate responses from the abundances of individual species to abundances of functional traits and test whether species that are near the edge of their tolerance to another stressor (in this case sedimentation) may exhibit stronger responses. The responses we observed were all nonlinear and some exhibited thresholds. While temperature was most frequently an important predictor, wave exposure and ENSO‐related variables were also frequently important and most ecological variables responded to interactions between environmental variables. There were also indications that species sensitive to another stressor responded more strongly to weaker climate‐related environmental change at the stressed site than the unstressed site. The observed interactions between climate variables, effects on key species or functional traits, and synergistic effects of additional anthropogenic stressors have important implications for understanding and predicting the ecological consequences of climate change to coastal ecosystems.     

Phylogeography and historical ecology of the North Atlantic intertidal

Abstract Recent glaciation covered the full extent of rocky intertidal habitat along the coasts of New England and the Canadian Maritimes. To test whether this glaciation in fact caused wholesale extinction of obligate rocky intertidal invertebrates, and thus required a recolonization from Europe, we compared American and European populations using allelic diversity and techniques adapted from coalescent theory. Mitochondrial DNA sequences were collected from amphi-Atlantic populations of three cold-temperate obligate rocky intertidal species (a barnacle, Semibalanus balanoides , and two gastropods, Nucella lapillus and Littorina obtusata ) and three cold-temperate habitat generalist species (a seastar, Asterias rubens ; a mussel, Mytilus edulis , and an isopod, Idotea balthica ). For many of these species we were able to estimate the lineage-specific mutation rate based on trans-Arctic divergences between Pacific and Atlantic taxa. These data indicate that some obligate rocky intertidal taxa have colonized New England from European populations. However, the patterns of persistence in North America indicate that other life-history traits, including mechanisms of dispersal, may be more important for surviving dramatic environmental and climatic change.     

Climate‐mediated changes in marine ecosystem regulation during El Niño

The degree to which ecosystems are regulated through bottom‐up, top‐down, or direct physical processes represents a long‐standing issue in ecology, with important consequences for resource management and conservation. In marine ecosystems, the role of bottom‐up and top‐down forcing has been shown to vary over spatio‐temporal scales, often linked to highly variable and heterogeneously distributed environmental conditions. Ecosystem dynamics in the Northeast Pacific have been suggested to be predominately bottom‐up regulated. However, it remains unknown to what extent top‐down regulation occurs, or whether the relative importance of bottom‐up and top‐down forcing may shift in response to climate change. In this study, we investigate the effects and relative importance of bottom‐up, top‐down, and physical forcing during changing climate conditions on ecosystem regulation in the Southern California Current System (SCCS) using a generalized food web model. This statistical approach is based on nonlinear threshold models and a long‐term data set (~60 years) covering multiple trophic levels from phytoplankton to predatory fish. We found bottom‐up control to be the primary mode of ecosystem regulation. However, our results also demonstrate an alternative mode of regulation represented by interacting bottom‐up and top‐down forcing, analogous to wasp‐waist dynamics, but occurring across multiple trophic levels and only during periods of reduced bottom‐up forcing (i.e., weak upwelling, low nutrient concentrations, and primary production). The shifts in ecosystem regulation are caused by changes in ocean‐atmosphere forcing and triggered by highly variable climate conditions associated with El Niño. Furthermore, we show that biota respond differently to major El Niño events during positive or negative phases of the Pacific Decadal Oscillation (PDO), as well as highlight potential concerns for marine and fisheries management by demonstrating increased sensitivity of pelagic fish to exploitation during El Niño.     

The influence of coastal upwelling on the functional structure of rocky intertidal communities

Summary Relationships between organisms at all trophic levels are influenced by the primary productivity of the ecosystem, and factors which enhance rates of primary production may modify trophic relationships and community structure. Nutrient enrichment of intertidal and nearshore waters leads to enhanced production by intertidal algae, and it was hypothesized that where rocky shores are washed by nutrient-rich upwelled waters, the intertidal communities should show a characteristic functional structure, based on the effects of enhanced primary production. Study sites were chosen on rocky shores in southern Africa, central Chile and the Canary Islands, in areas with and without coastal upwelling, and mid-shore community structure at these sites was analysed in terms of the abundance of certain functional guilds of organisms.It was found that algal cover and the biomass of herbivorous limpets supported per unit area on rocky shores were significantly greater in regions of coastal upwelling than in regions where upwelling did not occur. Ground cover by sessile filter-feeding organisms was significantly greater on shores in non-upwelled areas. However, correspondence analysis showed no functional aspect of intertidal community structure that was characteristic of coasts washed by upwelled waters. Primary reasons for this are probably the large variations in the nature of nutrient enrichment that accompanies upwelling, and in the nutrient status of non-upwelled areas. Other factors are man's exploitation of intertidal organisms and differences in the genetic origins of the intertidal species involved.