Kelps may compensate for low nitrate availability by using regenerated forms of nitrogen, including urea and ammonium.

Nitrate, the form of nitrogen often associated with kelp growth, is typically low in summer during periods of high macroalgal growth. More ephemeral, regenerated forms of nitrogen, such as ammonium and urea, are much less studied as sources of nitrogen for kelps, despite the relatively high concentrations of regenerated nitrogen found in the Southern California Bight, where kelps are common. To assess how nitrogen uptake by kelps varies by species and nitrogen form in southern California, USA, we measured uptake rates of nitrate, ammonium, and urea by Macrocystis pyrifera and Eisenia arborea individuals from four regions characterized by differences in nitrogen availability-Orange County, San Pedro, eastern Santa Catalina Island, and western Santa Catalina Island-during the summers of 2021 and 2022. Seawater samples collected at each location showed that overall nitrogen availability was low, but ammonium and urea were often more abundant than nitrate. We also quantified the internal %nitrogen of each kelp blade collected, which was positively associated with ambient environmental nitrogen concentrations at the time of collection. We observed that both kelp species readily took up nitrate, ammonium, and urea, with M. pyrifera taking up nitrate and ammonium more efficiently than E. arborea. Urea uptake efficiency for both species increased as internal percent nitrogen decreased. Our results indicate that lesser-studied, more ephemeral forms of nitrogen can readily be taken up by these kelps, with possible upregulation of urea uptake as nitrogen availability declines.

The functional effects of a dominant consumer are altered following the loss of a dominant producer.

Human impacts on ecosystems are resulting in unprecedented rates of biodiversity loss worldwide. The loss of species results in the loss of the multiple roles that each species plays or functions (i.e., "ecosystem multifunctionality") that it provides. A more comprehensive understanding of the effects of species on ecosystem multifunctionality is necessary for assessing the ecological impacts of species loss. We studied the effects of two dominant intertidal species, a primary producer (the seaweed Neorhodomela oregona) and a consumer (the shellfish Mytilus trossulus), on 12 ecosystem functions in a coastal ecosystem, both in undisturbed tide pools and following the removal of the dominant producer. We modified analytical methods used in biodiversity-multifunctionality studies to investigate the potential effects of individual dominant species on ecosystem function. The effects of the two dominant species from different trophic levels tended to differ in directionality (+/-) consistently (92% of the time) across the 12 individual functions considered. Using averaging and multiple threshold approaches, we found that the dominant consumer-but not the dominant producer-was associated with ecosystem multifunctionality. Additionally, the relationship between abundance and multifunctionality differed depending on whether the dominant producer was present, with a negative relationship between the dominant consumer and ecosystem function with the dominant producer present compared to a non-significant, positive trend where the producer had been removed. Our findings suggest that interactions among dominant species can drive ecosystem function. The results of this study highlight the utility of methods previously used in biodiversity-focused research for studying functional contributions of individual species, as well as the importance of species abundance and identity in driving ecosystem multifunctionality, in the context of species loss.

Biological modification of coastal pH depends on community composition and time.

Biological processes play important roles in determining how global changes manifest at local scales. Primary producers can absorb increased CO2 via daytime photosynthesis, modifying pH in aquatic ecosystems. Yet producers and consumers also increase CO2 via respiration. It is unclear whether biological modification of pH differs across the year, and, if so, what biotic and abiotic drivers underlie temporal differences. We addressed these questions using the intensive study of tide pool ecosystems in Alaska, USA, including quarterly surveys of 34 pools over 1 year and monthly surveys of five pools from spring to fall in a second year. We measured physical conditions, community composition, and changes in pH and dissolved oxygen during the day and night. We detected strong temporal patterns in pH dynamics. Our measurements indicate that pH modification varies spatially (between tide pools) and temporally (across months). This variation in pH dynamics mirrored changes in dissolved oxygen and was associated with community composition, including both relative abundance and diversity of benthic producers and consumers, whose role differed across the year, particularly at night. These results highlight the importance of the time of year when considering the ways that community composition influences pH conditions in aquatic ecosystems.

Accounting for variation in temperature and oxygen availability when quantifying marine ecosystem metabolism.

It is critical to understand how human modifications of Earth's ecosystems are influencing ecosystem functioning, including net and gross community production (NCP and GCP, respectively) and community respiration (CR). These responses are often estimated by measuring oxygen production in the light (NCP) and consumption in the dark (CR), which can then be combined to estimate GCP. However, the method used to create "dark" conditions-either experimental darkening during the day or taking measurements at night-could result in different estimates of respiration and production, potentially affecting our ability to make integrative predictions. We tested this possibility by measuring oxygen concentrations under daytime ambient light conditions, in darkened tide pools during the day, and during nighttime low tides. We made measurements every 1-3 months over one year in southeastern Alaska. Daytime respiration rates were substantially higher than those measured at night, associated with higher temperature and oxygen levels during the day and leading to major differences in estimates of GCP calculated using daytime versus nighttime measurements. Our results highlight the potential importance of measuring respiration rates during both day and night to account for effects of temperature and oxygen-especially in shallow-water, constrained systems-with implications for understanding the impacts of global change on ecosystem metabolism.

Climate Change Amelioration by Marine Producers: Does Dominance Predict Impact?

AbstractClimate change threatens biodiversity worldwide, and assessing how those changes will impact communities will be critical for conservation. Dominant primary producers can alter local-scale environmental conditions, reducing temperature via shading and mitigating ocean acidification via photosynthesis, which could buffer communities from the impacts of climate change. We conducted two experiments on the coast of southeastern Alaska to assess the effects of a common seaweed species, Neorhodomela oregona, on temperature and pH in field tide pools and tide pool mesocosms. We found that N. oregona was numerically dominant in this system, covering >60% of habitable space in the pools and accounting for >40% of live cover. However, while N. oregona had a density-dependent effect on pH in isolated mesocosms, we did not find a consistent effect of N. oregona on either pH or water temperature in tide pools in the field. These results suggest that the amelioration of climate change impacts in immersed marine ecosystems by primary producers is not universal and likely depends on species' functional attributes, including photosynthetic rate and physical structure, in addition to abundance or dominance.

Intertidal Canopy-forming Seaweeds Modulate Understory Seaweed Photoprotective Compounds.

Foundation species provide physical structure that enhances the diversity and abundance of associated organisms. Canopy-forming seaweeds are known to act as foundation species on rocky shores by lowering temperature and desiccation stress. Direct solar radiation, including ultraviolet (UV) light, can also reduce photosynthetic rates in algae, cause oxidative stress and DNA damage. The reduction in UV exposure provided by an algal canopy could therefore be important for understory organisms, including the red alga Chondrus crispus on New England's (USA) rocky shores, and this relationship may be more important at higher tidal elevations with increased low-tide exposure time. In field experiments, we investigated the relationship between tidal elevation and an index of C. crispus UV exposure, the concentration of UV-absorbing pigments. Low on the shore, C. crispus grew without a canopy. Higher on the shore, in the mid-intertidal zone, C. crispus was found under the canopy-forming rockweed, Fucus distichus subsp. evanescens. At this elevation, C. crispus was shaded (>50%; >1 m above MLLW). We performed a canopy removal experiment that spanned the mid-zone where C. crispus and F. distichus subsp. evanescens co-occur and the low-zone (no canopy). Following canopy removal in the mid-zone, UV pigment concentrations increased with tidal elevation. After accounting for the effect of elevation, removal of the algal canopy resulted in UV-protective pigment concentrations 2-fold higher than in un-manipulated control plots. These results suggest that amelioration of solar UV exposure might be another mechanism by which canopy seaweeds, acting as foundation species, facilitate understory seaweeds on rocky shores.

Flexibility of nutritional strategies within a mutualism: food availability affects algal symbiont productivity in two congeneric sea anemone species.

Mutualistic symbioses are common, especially in nutrient-poor environments where an association between hosts and symbionts can allow the symbiotic partners to persist and collectively out-compete non-symbiotic species. Usually these mutualisms are built on an intimate transfer of energy and nutrients (e.g. carbon and nitrogen) between host and symbiont. However, resource availability is not consistent, and the benefit of the symbiotic association can depend on the availability of resources to mutualists. We manipulated the diets of two temperate sea anemone species in the genus Anthopleura in the field and recorded the responses of sea anemones and algal symbionts in the family Symbiodiniaceae to our treatments. Algal symbiont density, symbiont volume and photosynthetic efficiency of symbionts responded to changes in sea anemone diet, but the responses depended on the species of sea anemone. We suggest that temperate sea anemones and their symbionts can respond to changes in anemone diet, modifying the balance between heterotrophy and autotrophy in the symbiosis. Our data support the hypothesis that symbionts are upregulated or downregulated based on food availability, allowing for a flexible nutritional strategy based on external resources.

Complementarity in spatial subsidies of carbon associated with resource partitioning along multiple niche axes.

The co-occurrence of several species, all of which share similar resource requirements, remains a paradox in ecology. Here, I evaluated resource use along multiple environmental axes to understand the potential for niche partitioning and complementarity in a guild of suspension-feeding rocky shore invertebrates. I focused on the mussels Perna canaliculus, Mytilus galloprovincialis, Aulacomya maoriana, and Xenostrobus pulex, all of which coexist along the coastline of the South Island of New Zealand. I quantified the mussels' distributions, both vertically on the shore and within the three-dimensional mussel bed matrix, and used carbon (δ13C) and nitrogen (δ15N) stable isotope ratios to compare species' diets. Mussels exhibited niche partitioning along all resource axes. Given the mussels' use of different spatial and food resources, I evaluated the potential for complementarity with respect to mussels' roles as mediators of an important spatial subsidy, carbon inputs from the nearshore ocean into rocky-shore ecosystems. In these systems, mussels are basal species, capturing and consuming particulate organic matter in the ocean and making it available for local consumption within the benthic community. I found that mussel diversity matters; even the most productive species-Perna canaliculus-only contributed about half of the mussel-mediated carbon that accumulated over a year. Multiple co-occurring species are, therefore, likely to provide more carbon at the base of the intertidal food web than any one species living independently, and complementarity in resource use along multiple environmental axes is likely an important mechanism underlying this relationship.

Interactive effects of large- and local-scale environmental gradients on phenotypic differentiation.

Intraspecific differentiation across a steep environmental gradient depends on the relative influences of evolutionary, organismal, and environmental processes. But steep environmental gradients may be nested within larger-scale, regional conditions that could influence these processes at the local scale. Therefore, we hypothesized that phenotypic differentiation along a steep environmental gradient would vary among regions. To test this hypothesis, we conducted a reciprocal transplant experiment on rocky intertidal shores, a habitat characterized by gradients in abiotic and biotic stress, in three regions of the Gulf of Maine. We used the ubiquitous and ecologically important rockweed species Fucus vesiculosus to quantify differentiation in growth, tissue nitrogen, and nitrogen productivity between upper and lower intertidal individuals. We found that phenotypic differentiation between tide heights varied among traits and regions. Although tissue nitrogen did not vary among any treatment combinations, growth and nitrogen productivity response were region specific. A strong effect of transplant height was found in all regions; however, an effect of home (source) height was only detectable in the central Gulf of Maine. Our study reveals that intraspecific responses to steep environmental gradients vary among populations, but the mechanisms underlying these patterns remain unknown. Given the roles that rockweeds play as food and habitat, these in situ patterns of growth and nitrogen productivity could have important community- and ecosystem-level consequences.

Primary producers may ameliorate impacts of daytime CO2 addition in a coastal marine ecosystem.

Predicting the impacts of ocean acidification in coastal habitats is complicated by bio-physical feedbacks between organisms and carbonate chemistry. Daily changes in pH and other carbonate parameters in coastal ecosystems, associated with processes such as photosynthesis and respiration, often greatly exceed global mean predicted changes over the next century. We assessed the strength of these feedbacks under projected elevated CO2 levels by conducting a field experiment in 10 macrophyte-dominated tide pools on the coast of California, USA. We evaluated changes in carbonate parameters over time and found that under ambient conditions, daytime changes in pH, pCO2, net ecosystem calcification (NEC), and O2 concentrations were strongly related to rates of net community production (NCP). CO2 was added to pools during daytime low tides, which should have reduced pH and enhanced pCO2. However, photosynthesis rapidly reduced pCO2 and increased pH, so effects of CO2 addition were not apparent unless we accounted for seaweed and surfgrass abundances. In the absence of macrophytes, CO2 addition caused pH to decline by ∼0.6 units and pCO2 to increase by ∼487 µatm over 6 hr during the daytime low tide. As macrophyte abundances increased, the impacts of CO2 addition declined because more CO2 was absorbed due to photosynthesis. Effects of CO2addition were, therefore, modified by feedbacks between NCP, pH, pCO2, and NEC. Our results underscore the potential importance of coastal macrophytes in ameliorating impacts of ocean acidification.

Stoichiometric Mismatch between Consumers and Resources Mediates the Growth of Rocky Intertidal Suspension Feeders.

The concept of ecological stoichiometry-the balancing of elemental ratios in ecological interactions-has transformed our thinking about processes in natural systems. Here, this perspective is applied to rocky shore ecosystems to explore the consequences of variation in internal nutrient ratios across two trophic levels. Specifically, I measured the internal concentrations of carbon (C) and nitrogen (N) in mussels (Mytilus spp.) and particulate organic matter (POM) to evaluate the effects of stoichiometric mismatch-the difference in the carbon-to-nitrogen ratio (C:N) between a consumer and its resources-on mussel growth at sites on the coasts of Oregon, USA, and the South Island of New Zealand. As POM quality (i.e., Chl a, a proxy for phytoplankton availability in the POM) increased, C:N of the POM declined, but C:N of mussels increased. This resulted in a greater mismatch in C:N between mussels and their food source at low Chl a. Mussel growth across sites was positively associated with Chl a, particulate organic carbon (POC), and particulate organic nitrogen (PON) but negatively associated with stoichiometric mismatch. Overall, as the elemental ratios of consumers became more different from those of their resources, growth declined, likely due to the energetic cost associated with processing lower quality food. Furthermore, the effect of food quantity on growth depended on stoichiometric mismatch. In New Zealand, where mismatch was high-i.e., consumer C:N differed substantially from resource C:N-consumer growth was strongly affected by resource quantity (Chl a or POC). However, in Oregon, where mismatch was low, the relationship between resource quantity and growth was considerably weaker. This interaction between resource quantity and mismatch was not apparent for PON, which is consistent with variation in PON underlying variation in POM C:N and highlights the role of N in limiting growth. Previous research has neglected the importance of ecological stoichiometry as a mediator of consumer-resource interactions in rocky intertidal communities. I show that resource quality and quantity interact to determine consumer growth, highlighting the utility of ecological stoichiometry in understanding spatial subsidies in benthic marine systems.

Spatial scale mediates the effects of biodiversity on marine primary producers.

Most studies evaluating the effects of biodiversity on ecosystem functioning are conducted at a single location, limiting our understanding of how diversity-function relationships may change when measured across different spatial scales. We conducted a species-removal experiment at three sites nested in each of three regions along the rocky intertidal coastline of the Gulf of Maine, USA, to evaluate the potential for scale-dependent effects of species loss on the biomass of intertidal seaweed assemblages. We randomly assigned 50 plots in the mid-intertidal zone at each site to one of five treatments (n = 10 plots each): an unmanipulated control, a polyculture plot that contained our three target seaweed species, and three monoculture plots. We manipulated diversity by removing all non-target species from monoculture and polyculture plots, then removing additional biomass from polyculture plots, proportionate to species' relative abundances, so that the average amount removed from monoculture and polyculture plots was equivalent at each site. At the largest spatial scale, all sites considered together, after accounting for region and site nested within region seaweed diversity had consistent, positive effects on seaweed cover. Diverse polyculture plots always had higher cover than was predicted by the average performance of the component seaweed species and usually had higher cover than even the best-performing component species. Diversity effects weakened and became less consistent at smaller spatial scales, so that at the scale of individual sites, diverse polycultures only performed better than the average of monocultures ~40% of the time. Hence, our results indicate that weak and/or inconsistent biodiversity effects at the level of individual sites may scale up to stronger, more consistent effects at larger spatial scales. Quantitative summaries of biodiversity experiments conducted at the scale of individual sites do not capture this spatial aspect of biodiversity effects and may therefore underestimate the functional consequences of biodiversity loss.

Nitrate uptake varies with tide height and nutrient availability in the intertidal seaweed Fucus vesiculosus.

Intertidal seaweeds must cope with a suite of stressors imposed by aerial exposure at low tide, including nutrient limitation due to emersion. Seaweeds can access nutrients only when submerged, so individuals living higher compared to lower on the shore may have adaptations allowing them to acquire sufficient amounts of nutrients to survive and maintain growth. Using a combination of observations and experiments, we aimed to identify intraspecific variation in nitrate uptake rates across the intertidal distribution of F. vesiculosus, as well as test for acclimation in response to a change in tide height. We replicated our study at sites spanning nearly the entire Gulf of Maine coastline, to examine how local environmental variability may alter intraspecific variation in nitrate uptake. We found that average nitrate uptake rates were ~18% higher in upper compared to lower intertidal Fucus vesiculosus. Furthermore, we found evidence for both acclimation and adaptation to tide height during a transplant experiment. F. vesiculosus transplanted from the lower to the upper intertidal zone was characterized by increased nitrate uptake, but individuals transplanted from the upper to the lower intertidal zone retained high uptake rates. Our observations differed among Gulf of Maine regions and among time points of our study. Importantly, these differences may reflect associations between nitrate uptake rates and abiotic environmental conditions and seaweed nutrient status. Our study highlights the importance of long-term variation in ambient nutrient supply in driving intraspecific variation of seaweeds across the intertidal gradient and local and seasonal variation in ambient nutrient levels in mediating intraspecific differences.

Warming and Elevated CO2 Interact to Drive Rapid Shifts in Marine Community Production.

Predicting the outcome of future climate change requires an understanding of how alterations in multiple environmental factors manifest in natural communities and affect ecosystem functioning. We conducted an in situ, fully factorial field manipulation of CO2 and temperature on a rocky shoreline in southeastern Alaska, USA. Warming strongly impacted functioning of tide pool systems within one month, with the rate of net community production (NCP) more than doubling in warmed pools under ambient CO2 levels relative to initial NCP values. However, in pools with added CO2, NCP was unaffected by warming. Productivity responses paralleled changes in the carbon-to-nitrogen ratio of a red alga, the most abundant primary producer species in the system, highlighting the direct link between physiology and ecosystem functioning. These observed changes in algal physiology and community productivity in response to our manipulations indicate the potential for natural systems to shift rapidly in response to changing climatic conditions and for multiple environmental factors to act antagonistically.

Community context mediates the top-down vs. bottom-up effects of grazers on rocky shores.

Interactions between grazers and autotrophs are complex, including both top-down consumptive and bottom-up facilitative effects of grazers. Thus, in addition to consuming autotrophs, herbivores can also enhance autotroph biomass by recycling limiting nutrients, thereby increasing nutrient availability. Here, we evaluated these consumptive and facilitative interactions between snails (Littorina littorea) and seaweeds (Fucus vesiculosus and Ulva lactuca) on a rocky shore. We partitioned herbivores' total effects on seaweeds into their consumptive and facilitative effects and evaluated how community context (the presence of another seaweed species) modified the effects of Littorina on a focal seaweed species. Ulva, the more palatable species, enhanced the facilitative effects of Littorina on Fucus. Ulva did not modify the consumptive effect of Littorina on Fucus. Taken together, the consumptive and facilitative effects of snails on Fucus in the presence of Ulva balanced each other, resulting in no net effect of Littorina on Fucus. In contrast, the only effect of Fucus on Ulva was to enhance consumptive effects of Littorina on Ulva. Our results highlight the necessity of considering both consumptive and facilitative effects of herbivores on multiple autotroph species in order to gain a mechanistic understanding of grazers' top-down and bottom-up roles in structuring communities.

Plant-animal diversity relationships in a rocky intertidal system depend on invertebrate body size and algal cover.

Considerable research has examined the influence of herbivores on the maintenance of plant diversity, but fewer studies have examined the reciprocal effect of plant diversity on the animals that use the plant community for food and shelter, particularly in marine systems. Several mechanisms could underlie such effects. Animal diversity and abundance could be increased by complementary use of different plants by different animals, or by an indirect effect of plant diversity on plant production that results in more total plant biomass in high plant-diversity communities. Alternatively, plant species identity could play a dominant role leading to sampling effects or no effect of diversity at all. We conducted a six-year field manipulation of the richness of rocky shore seaweeds in northern California and measured the effects of algal richness and identity on the invertebrate community, from meiofauna to macrofauna. We found that diverse algal communities hosted more species of both large and small invertebrates than the average algal monoculture but that the mechanisms underlying this pattern differed substantially for organisms of different size. More species of macrofauna occurred in the polycultures than in any of the monocultures, likely due to the greater total cover of algae produced in polycultures. Rare and common macrofaunal taxa responded to host plant species richness in opposite ways, with more occurrences of rare taxa and lower abundance of very common taxa in the polycultures. In contrast, meiofaunal richness in polycultures was no different than that of monocultures of finely branched species, leading to strong effects of algal identity. Our findings are similar to those from terrestrial systems in that the effects of plant diversity we observed were most related to the greater amount of habitat in polycultures as a result of overyielding in algal biomass. However, our findings differ from those in terrestrial systems in that the primary mechanisms for both richness and identity effects appear related to the value of plants as shelter from harsh abiotic conditions or predation rather than food, and in that animal body size altered the mechanisms underlying diversity effects.

Nitrogen availability limits phosphorus uptake in an intertidal macroalga.

Nutrients such as nitrogen (N) and phosphorus (P) limit primary productivity, and recent anthropogenic activities are changing the availability of these nutrients, leading to alterations in the type and magnitude of nutrient limitation. Recent work highlights the potential for N and P to interact to limit primary production in terrestrial and freshwater systems. However, mechanisms underlying co-limitation are not well described. Documentation of ambient nutrient levels and tissue nutrients of the intertidal macroalga Fucus vesiculosus for 2 years in the southern Gulf of Maine, USA, indicates that N availability may be impacting the ability of F. vesiculosus to access P, despite relatively high ambient P concentrations. To experimentally validate these observations, F. vesiculosus individuals were enriched with N or P for 6 weeks, followed by an uptake experiment to examine how the interactions between these nutrients affected macroalgal N and P uptake efficiency. Results illustrate that exposure of seaweed to different nutrient regimes influenced nutrient uptake efficiency. Notably, seaweeds enriched with N displayed the highest P uptake efficiency at low, biologically relevant, P concentrations. Our results confirm that N availability may be mediating the ability of primary producers to access P. These interactions between limiting nutrients have implications for not only the growth and functioning of primary producers who rely directly on these nutrients but also the entire communities that they support.

Realistic changes in seaweed biodiversity affect multiple ecosystem functions on a rocky shore.

Given current threats to biodiversity, understanding the effects of diversity changes on the functions and services associated with intact ecosystems is of paramount importance. However, limited realism in most biodiversity studies makes it difficult to link the large and growing body of evidence for important functional consequences of biodiversity change to real-world losses of biodiversity. Here, we explored two methods of incorporating realism into biodiversity research: (1) the use of two-, five-, and eight-species assemblages that mimicked those that we observed in surveys of seaweed biodiversity patterns on a northern California (USA) rocky shore and the explicit comparison of those assemblages to random assemblages compiled from the same local species pool; and (2) the measurement of two fundamental ecosystem functions, nitrate uptake and photosynthesis, both of which contribute to growth of primary producers. Specifically, we measured nitrate uptake rates of seaweed assemblages as a function of initial nitrate concentrations and photosynthetic rates as a function of irradiance levels for both realistic and random assemblages of seaweeds. We only observed changes in ecosystem functioning along a richness gradient for realistic assemblages, and both maximum nitrate uptake rates (V(max)) and photosynthetic light use efficiency values (alpha(p) = P(max)/I(K)) were higher in realistic assemblages than in random assemblages. Furthermore, the parameter affected by changes in richness depended on the function being measured. Both V(max) and alpha(p) declined with increasing richness in nonrandom assemblages due to a combination of species identity effects (for V(max) and overyielding effects (for both V(max) and alpha(p)). In contrast, neither nitrate uptake efficiency values (alpha(N) = V(max)/K(s)), nor maximum photosynthetic rates (Pmax) changed along the gradient in seaweed species richness. Furthermore, overyielding was only evident in realistic assemblages, and the parameters exhibiting overyielding, including V(max), alpha(N), P(max), and alpha(p), changed along a gradient in species richness. Our results suggest that in realistic assemblages of species (1) some functions may be maximized at low levels of species richness, and (2) it is not only diversity, per se, that is important for sustaining multiple ecosystem functions, but also the range of diversity values in an ecosystem.

Additive effects of physical stress and herbivores on intertidal seaweed biodiversity.

Patterns in rocky intertidal seaweed biodiversity influence the resilience and functioning of these important primary producer communities. In turn, seaweed biodiversity patterns are the result of many ecological factors. We determined the influences of thermal and desiccation stress, herbivory, and nutrients on seaweed biodiversity on a northern California rocky shoreline. In a fully crossed design at two tidal heights at wave-protected and exposed sites, we deployed screens to reduce stress, removed herbivores, and added nutrients for 18 months. The treatments reduced temperature, increased relative humidity, decreased herbivore abundances, and increased nitrogen in both seawater and seaweeds. Seaweed abundance and biodiversity (cover, biomass, species richness, diversity, evenness, and community composition) were influenced by tidal height, physical stress, and herbivores. Wave exposure affected all response variables except biomass and evenness. Stress and herbivores had independent additive effects on seaweed abundance and diversity. Physical stress did not make the community as a whole more susceptible to herbivores, and screens had overarching positive effects on seaweed biodiversity even though they also had positive effects on herbivore abundance. Nutrients had virtually no effect on seaweed biodiversity, and we observed no bottom-up effects of nutrient addition on herbivore density or biomass. Small green algae and diatoms were important contributors to overall algal cover and to changes in composition across treatments, but larger macroalgae dominated the species richness response. The striking absence of interactions between stress and herbivory highlights how seaweed communities can respond independently to important drivers of biodiversity. Thus, nonadditive, potentially synergistic effects do not necessarily complicate the understanding of how seaweed biodiversity responds to environmental change.

Invasion of the red seaweed Heterosiphonia japonica spans biogeographic provinces in the Western North Atlantic Ocean.

The recent invasion of the red alga Heterosiphonia japonica in the western North Atlantic Ocean has provided a unique opportunity to study invasion dynamics across a biogeographical barrier. Native to the western North Pacific Ocean, initial collections in 2007 and 2009 restricted the western North Atlantic range of this invader to Rhode Island, USA. However, through subtidal community surveys, we document the presence of Heterosiphonia in coastal waters from Maine to New York, USA, a distance of more than 700 km. This geographical distribution spans a well-known biogeographical barrier at Cape Cod, Massachusetts. Despite significant differences in subtidal community structure north and south of Cape Cod, Heterosiphonia was found at all but two sites surveyed in both biogeographic provinces, suggesting that this invader is capable of rapid expansion over broad geographic ranges. Across all sites surveyed, Heterosiphonia comprised 14% of the subtidal benthic community. However, average abundances of nearly 80% were found at some locations. As a drifting macrophyte, Heterosiphonia was found as intertidal wrack in abundances of up to 65% of the biomass washed up along beaches surveyed. Our surveys suggest that the high abundance of Heterosiphonia has already led to marked changes in subtidal community structure; we found significantly lower species richness in recipient communities with higher Heterosiphona abundances. Based on temperature and salinity tolerances of the European populations, we believe Heterosiphonia has the potential to invade and alter subtidal communities from Florida to Newfoundland in the western North Atlantic.