Abstract. Ocean acidification, chemical changes to the carbonate system of seawater, is emerging as a key environmental challenge accompanying global warming and other humaninduced perturbations. Considerable research seeks to define the scope and character of potential outcomes from this phenomenon, but a crucial impediment persists. Ecological theory, despite its power and utility, has been only peripherally applied to the problem. Here we sketch in broad strokes several areas where fundamental principles of ecology have the capacity to generate insight into ocean acidification's consequences. We focus on conceptual models that, when considered in the context of acidification, yield explicit predictions regarding a spectrum of population-and community-level effects, from narrowing of species ranges and shifts in patterns of demographic connectivity, to modified consumer-resource relationships, to ascendance of weedy taxa and loss of species diversity. Although our coverage represents only a small fraction of the breadth of possible insights achievable from the application of theory, our hope is that this initial foray will spur expanded efforts to blend experiments with theoretical approaches. The result promises to be a deeper and more nuanced understanding of ocean acidification and the ecological changes it portends.

The e ects of ocean acidification (OA) on the structure and complexity of coastal marine biogenic habitat have been broadly overlooked. Here we explore how declining pH and carbonate saturation may a ect the structural complexity of four major biogenic habitats. Our analyses predict that indirect e ects driven by OA on habitat-forming organisms could lead to lower species diversity in coral reefs, mussel beds and some macroalgal habitats, but increases in seagrass and other macroalgal habitats. Available in situ data support the prediction of decreased biodiversity in coral reefs, but not the prediction of seagrass bed gains. Thus, OA-driven habitat loss may exacerbate the direct negative e ects of OA on coastal biodiversity; however, we lack evidence of the predicted biodiversity increase in systems where habitat-forming species could benefit from acidification. Overall, a combination of direct e ects and community-mediated indirect e ects will drive changes in the extent and structural complexity of biogenic habitat, which will have important ecosystem e ects. Supplementary Fig. 1). and species richness in tropical coral reefs (Fig. 1b) leads to the 38 prediction that species richness will decline with expected changes 39 in carbonate chemistry associated with OA (Fig. 1c). 40In mussel beds of the US Pacific Northwest, the percentage of 41 cover of large Mytilus mussels is projected to decline with declining 42 pH, to be replaced by species that lack the structural complexity

This study investigated (1) the effects of salinity and temperature on the bacterial community composition of developing biofilms, and (2) the responses of marine invertebrate larvae (the polychaete Hydroides elegans and the barnacles Balanus amphitrite and B. trigonus) to these biofilms during settlement (i.e. attachment to a surface and metamorphosis into juveniles). Biofilms developed in a 3 × 3 array of salinity and temperature treatments resulted in different bacterial community compositions (revealed by DGGE and T-RFLP), bacterial densities and total biomasses. Larval settlement of B. amphitrite and B. trigonus was induced by biofilms developed at high temperatures (23 and 30°C), but was unaffected (B. amphitrite) or inhibited (B. trigonus) by those developed at a low temperature (16°C). The settlement response of these barnacles did not correlate with the biomass or the bacterial density of the biofilms, but did coincide with the marked differences in bacterial community composition between the biofilms developed at different temperatures. In contrast, larval settlement of H. elegans differed slightly among biofilms developed in different salinities, but not among those developed at different temperatures. This settlement response was moderately correlated with bacterial density but had no apparent relationship with bacterial community composition of the biofilms. Our results implied that the community composition and cell density of bacteria in biofilms, which can vary with local environmental conditions, may allow larvae of the 2 barnacles and H. elegans, respectively, to distinguish between habitats with different environmental conditions. KEY WORDS: Bacterial community · Barnacle · Biofilm · DGGE · Larval settlement · Polychaete · T-RFLP Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 38: [41][42][43][44][45][46][47][48][49][50][51] 2005 Bourget 1988, Pearce & Scheibling 1991, Keough & Raimondi 1996. This subtle response is believed to be mediated by the structural and/or physiological attributes of biofilms, which may vary spatially and temporally according to the biotic (e.g. availability and physiology of colonizing species, competition and cooperation among species, and grazing pressure) and abiotic factors (e.g. latitude, depth, illumination, exposure time, season, water chemistry, nutrient supply, flow shear and substratum characteristics) of the local environment (Wieczorek & Todd 1998).Despite decades of study, the bioactive components of biofilms that mediate larval settlement are still largely unknown. Many investigations have nevertheless highlighted the bacterial component of biofilms as a major mediator for larval settlement of many invertebrates (Holmström et al. 1992, Harder et al. 2002. In laboratory assays, monospecies films of bacteria can influence larval settlement in a variety of ways (from inhibition to neutral to induction), depending on the species of bacteria and larvae of concern. The specificity of larval response ...

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