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.

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.

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.

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.

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.

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.

The loss of consumers threatens the integrity of ecological systems, but the mechanisms underlying the effects on communities and ecosystems remain difficult to predict. This is, in part, due to the complex roles that consumers play in those systems. Here, we highlight this complexity by quantifying two mechanisms by which molluscan grazers-typically thought of as consumers of their algal resources-facilitate algae on rocky shores. Initial observations in high-zone tide pools revealed that both water-column ammonium concentrations and photosynthetic biomass were higher in pools containing higher densities of grazers, suggesting that local-scale nutrient recycling by the grazers could be enhancing algal biomass. We assessed this possibility by experimentally manipulating grazer abundances at the level of whole tide pools but controlling access of those grazers to experimental plots within each pool. Contrary to predictions that algal biomass inside grazer exclusions would increase as grazer abundances in the pools increased, we found that algal biomass inside grazer-exclusion fences was unaffected by grazer abundances. Instead, the consumptive effects of grazers that were evident at low grazer abundances transitioned to facilitative effects as experimentally manipulated grazer abundances increased. This finding suggested that these positive interactions were associated with the physical presence of grazers and not just grazers' effects on nutrient availability. Subsequent experiments highlighted the potential role of "slime"-the pedal mucous trails left behind as the mollusks crawl on the substratum-in promoting the recruitment of algae and thereby mediating a spatial subsidy of new organic matter into the system. Furthermore, different grazer groups contributed disproportionately to ammonium excretion (i.e., turban snails) versus slime production (i.e., littorine snails), suggesting a potential role for grazer diversity. Our work highlights the complex ways in which consumers affect their resources, including multiple, complementary mechanisms by which these grazers facilitate the algae they consume.

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.

This study shows that, even on exposed, wave-swept, rocky shores in a nutrient-replete upwelling ecosystem, mussels (Mytilus californianus) facilitate the growth of the seaweed Porphyra perforata by enhancing nutrient concentrations in the nearby water column. In field surveys on emergent substrate in the mid-intertidal zone, we found ten times greater abundance of P. perforata on mussels than on adjacent rock. In field experiments, P. perforata accumulated and grew more quickly on mussels than on bare rock or on mussel mimics, suggesting that nutrients excreted by mussels might be responsible for greater P. perforata cover. At high tide, water column ammonium concentrations over mussel beds were nearly double those found over bare rock. Correspondingly, tissue nitrogen concentrations were higher, and carbon-to-nitrogen ratios were lower in P. perforata growing on mussels compared to bare rock. Given the dominance of mussels in mid-intertidal regions of temperate coasts worldwide, ammonium regeneration could be a general contributor to local-scale nutrient availability, even in high-flow systems characterized by high nutrient concentrations. © 2009, by the American Society of Limnology and Oceanography, Inc.

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.