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Recent large-scale seagrass declines have prompted experimental investigations of potential mechanisms. Although many studies have implicated eutrophication or reductions of epiphyte grazers in these declines, few experiments have simultaneously manipulated both factors to assess their relative effects. This study used meta-analyses of 35 published seagrass studies to compare the relative strength of 'top-down' grazer effects and 'bottom-up' nutrient effects on epiphyte biomass and seagrass above-ground growth rate, above-ground biomass, below-ground biomass, and shoot density. A surprising result was that seagrass growth and biomass were limited in situ by sediment nutrients; light limitation has been emphasized in the literature to date. Water column enrichments, which were correlated with increased epiphyte biomass, had strong negative effects on seagrass biomass. Grazers overall had a positive effect on shoot density, but negligible effects on seagrass biomass and growth rate. However, analyzing epiphyte grazers separately from other grazers revealed positive effects of grazing on seagrass response variables and corresponding negative impacts on epiphyte biomass. The positive effects of epiphyte grazers were comparable in magnitude to the negative impacts of water column nutrient enrichment, suggesting that the 2 factors should not be considered in isolation of each other. Until the determinants of epiphyte grazer populations are empirically examined, it will be difficult to address the contribution that overfishing and cascading trophic effects have had on seagrass decline. Because increases in water column nutrients are documented in many regions, efforts to reduce coastal eutrophication are an appropriate and necessary focus for the management and conservation of seagrass ecosystems.

North American green sturgeon (Acipenser medirostris) display the distinctive behavior of long ocean migrations along the west coast punctuated by summer residence in estuaries; however, little is known about stock composition in these tidal environments. Pairwise comparisons and genetic clustering analysis were used to characterize the two green sturgeon Distinct Population Segments (DPSs) from 20 collections using eight tetrasomic and two disomic microsatellite loci. The observed pattern of green sturgeon DPS composition among five estuaries in California, Oregon, and Washington was supported with assignment testing approaches utilizing the same genotypic data in codominant polysomic and pseudodominant allele phenotype formats. The majority of individuals in northern DPS estuaries originated from the threatened Southern DPS, except in Winchester Bay and Grays Harbor. We detected few Northern DPS green sturgeon in San Pablo Bay, the principal Southern DPS estuary, supporting that green sturgeon preferentially disperse north once they enter their coastal migration. Our genetic findings suggest that stock complexity in green sturgeon is pervasive and support precautionary, interjurisdictional approaches for managing green sturgeon beyond rigid, regulatory boundaries. L'esturgeon vert (Acipenser medirostris) d'Amerique du Nord possede un comportement particulier qui comprend de longues migrations en mer le long de la cote ouest interrompues par des sejours d'ete dans les estuaires; on connait cependant peu la composition des stocks dans ces milieux sous influence de la maree. Des comparaisons appariees et une analyse de groupement genetique nous ont servi a caracteriser deux segments distincts de population (DPS) d'esturgeons verts a partir de 20 echantillons et a l'etude de huit locus microsatellites tetrasomiques et deux disomiques. Le patron obtenu de composition des DPS dans cinq estuaires de Californie, d'Oregon et du Washington est confirme par des methodes d'essais d'attribution d'apres les memes donnees genotypiques dans des formats de phenotypes d'alleles polysomiques codominants et pseudodominants. La plupart des individus des estuaires du DPS du nord proviennent du DPS menace du sud, a l'exception de ceux de la baie Winchester et de Grays Harbor. Nous avons decele peu d'esturgeons verts du DPS du nord dans la baie de San Pablo, l'estuaire principal du DPS du sud, ce qui appuie l'hypothese qui veut que les sturgeons verts se dispersent de preference vers le nord une fois qu'ils commencent leur migration cotiere. Nos observations genetiques indiquent que la complexite du stock est generale et elles nous amenent a recommander des approches prudentes couvrant les diverses juridictions et depassant les frontieres rigides de la reglementation pour la gestion de l'esturgeon vert. [Traduit par la Redaction], Introduction Information about the stock composition of anadromous sturgeon aggregating in coastal bays and estuaries is very limited, yet essential for reducing risks associated with port dredging and development, coastal [...]