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Recent warming trends have driven widespread changes in the performance and distribution of species in many regions, with consequent shifts in assemblage structure and ecosystem functioning. However, as responses to warming vary across species and regions, novel communities are emerging, particularly where warm-affinity range-expanding species have rapidly colonized communities still dominated by cold-affinity species. Such community reconfiguration may alter core ecosystem processes, such as productivity or nutrient cycling, yet it remains unclear whether novel communities function similarly to those they have replaced, and how continued warming will alter functioning in the near future. Using simplified kelp forest communities as a model system, we compared rates of respiration, consumption and secondary productivity between current cold-affinity and future warm-affinity kelp assemblages under both present-day temperatures and near-future warming in a series of mesocosm experiments. Overall, respiration rates of gastropods and amphipods increased with warming but did not differ between cold and warm affinity kelp assemblages. Consumption rates of three consumers (urchin, gastropod and amphipod) differed between kelp assemblages but only amphipod consumption rates increased with warming. A diet derived from warm-affinity kelp assemblages led to a decrease in growth and biomass of urchins, whereas the response of other consumers was variable depending on temperature treatment. These results suggest that climate-driven changes in assemblage structure of primary producers will alter per capita rates of ecosystem functioning, and that specific responses may vary in complex and unpredictable ways, with some mediated by warming more than others. Understanding how differences in life history and functional traits of dominant species will affect ecological interactions and, in turn, important ecosystem processes is crucial to understanding the wider implications of climate-driven community reconfiguration.

Coastal habitats dominated by marine macroalgae typically exhibit high rates of primary productivity and play a key role in local and regional carbon cycles and stores. In temperate regions, large brown algae (i.e. kelps and fucoids) contribute significantly to macroalgal primary production, most of which is exported from source habitats as detritus. The ultimate fate of this detritus and the processes controlling detrital pathways into food webs and carbon cycles remain poorly understood. Based on field surveys, we quantified the biomass of kelp-derived detritus (wrack) at sandy and pebble-dominated shores in Ireland and conducted a manipulative field experiment to test for inter-specific differences in detritus degradation rates and the effect of macroinvertebrate detritivores. Overall, accumulated wrack biomass was similar on all shores but varied temporally depending on habitat type. Degradation rates and the nutritional (C:N) and chemical (polyphenol concentrations) properties differed among kelp species. Interestingly, exclusion of macroinvertebrate detritivores did not affect kelp degradation rates, C:N ratios or polyphenol content. Our findings show that rates of macroalgal breakdown differ among kelp species and that, in contrast to other aquatic systems, macroinvertebrates appear to play a very limited role in the breakdown of these marine detrital subsidies, suggesting a key role for meiofauna and microbes in detritus processing. Increasing recognition for the role of detritus in coastal food webs and carbon cycles warrants a better understanding of the mechanisms underpinning degradation rates.

Coastal habitats are increasingly recognized as fundamentally important components of global carbon cycles, but the rates of carbon flow associated with marine macrophytes are not well resolved for many species in many regions. We quantified density, rates of primary productivity, and detritus production of intertidal stands of two common intertidal kelp species—Laminaria digitata (oarweed) and Saccharina latissima (sugar kelp)—on four NE Atlantic rocky shores over 22 months. The density of L. digitata was greater at exposed compared to moderately exposed shores but remained consistently low for S. latissima throughout the survey period. Individual productivity and erosion rates of L. digitata did not differ between exposed and moderately exposed shores but differed across exposure levels throughout the year at moderately exposed sites only. Productivity and erosion of S. latissima remained low on moderately exposed shores and showed no clear seasonal pattern. Patterns of productivity and total detrital production (erosion and dislodgement) per m2 of both L. digitata and S. latissima followed closely that of densities per m2, peaking in May during both survey years. Temperature and light were key factors affecting the productivity rates of L. digitata and S. latissima. Erosion rates of L. digitata were affected by wave exposure, temperature, light, grazing, and epiphyte cover, but only temperature‐affected erosion of S. latissima. Production of biomass and detritus was greater in L. digitata than in S. latissima and exceeded previous estimates for subtidal and warmer‐water affinity kelp populations (e.g., Laminaria ochroleuca). These biogenic habitats are clearly important contributors to the coastal carbon cycle that have been overlooked previously and should be included in future ecosystem models. Further work is required to determine the areal extent of kelp stands in intertidal and shallow subtidal habitats, which is needed to scale up local production estimates to entire coastlines. Using long‐term field surveys, we present new data quantifying key processes (production and detrital production) in an intertidal kelp system under differing environmental conditions. We show that density and productivity of intertidal kelp differ between kelp species and levels of wave exposure and, as predicted, that rates of productivity and erosion vary seasonally. We also identify temperature, light, and wave exposure as primary factors affecting these key processes, along with grazing pressure and epiphyte cover. Finally, we show that per m2, production and detrital production of intertidal kelps is comparable to that of subtidal and range‐expanding warm‐temperate species, highlighting the important contribution of these habitats to the coastal carbon cycle that has been previously overlooked.