Effects of eutrophication, grazing, and algal blooms on rocky shores

Eutrophication can profoundly change rocky shore communities. These changes often cause the replacement of perennial, canopy-forming algae such as Fucus spp. with annual, bloom-forming algae such as Enteromorpha spp. Grazing, however, can counteract eutrophication by eliminating the annual algae�s susceptible recruits. We examine these generalizations across large scales. We use replicated ��bioassay�� experiments to compare the effects of eutrophication and grazing across four paired control versus eutrophied sites in the Northwest Atlantic and four eutrophied sites in the Baltic Sea in spring and summer. At each site, annual algal recruitment and grazing pressure were estimated using tiles seeded with Enteromorpha intestinalis propagules. Tiles were exposed for 3 weeks with grazers excluded or allowed access. Productivity of E. intestinalis recruits was strongly related to eutrophication (10-fold increase) and grazing (80% decrease) and was weakly related to season. While the absolute grazing rate increased in a linear fashion with algal productivity, the relative grazing rate remained surprisingly constant (;80%). Comparative field surveys showed that perennial algae decreased by 30-60%, while annual algae, filter feeders, and grazers increased across a gradient of eutrophication. As eutrophication increased from control to eutrophied to point source sites, rocky shore communities became increasingly dominated by single species of annual algae or filter feeders, and community diversity declined consistently by 24-46%. We conclude that grazers are important controllers of algal blooms but that, ultimately, they cannot override the effects of increasing eutrophication on rocky shore community structure and biodiversity

Long-Term Change in a Meso-Predator Community in Response to Prolonged and Heterogeneous Human Impact

Sharks and rays' abundance can decline considerably with fishing. Community changes, however, are more complex because of species interactions, and variable vulnerability and exposure to fishing. We evaluated long-term changes in the elasmobranch community of the Adriatic Sea, a heavily exploited Mediterranean basin where top-predators have been strongly depleted historically, and fishing developed unevenly between the western and eastern side. Combining and standardizing catch data from five trawl surveys from 1948 -- 2005, we estimated abundance trends and explained community changes using life histories, fish-market and effort data, and historical information. We identified a highly depleted elasmobranch community. Since 1948, catch rates have declined by .94% and 11 species ceased to be detected. The exploitation history and spatial gradients in fishing pressure explained most patterns in abundance and diversity, including the absence of strong compensatory increases. Ecological corridors and large-scale protected areas emerged as potential management options for elasmobranch conservation

Shark Declines in the Mediterranean Sea

Summarizes a study of population and biomass trends of large sharks in the Mediterranean, and highlights the risk of some species becoming extinct as a result of unintended capture in fishing gear, targeted shark fishing, and human population pressure

Coastal food web structure, carbon storage, and nitrogen retention regulated by consumer pressure and nutrient loading

By factorial field experiments we analyzed the relative effects of increased nutrient (N+P) loading and natural grazing pressure on species composition, carbon storage, and nitrogen retention in the Baltic Sea littoral food web, composed of macroalgae, grazers (snails, isopods, amphipods), and predators (shrimps, crabs, fish). Nitrogen was depleted relative to phosphorus throughout most of the year. Increasing nitrogen (6–200% over ambient concentrations) enhanced algal productivity and cover of fast-growing annual algae, grazer, and predator densities, suggesting a three-level bottom-up effect. With increasing nitrogen loading, annual algae increasingly blocked perennial algal recruitment (65–98% decrease) and growth. Grazers counteracted the effects of nutrient enrichment on algal species composition through selective consumption of annual algae. Grazer exclusion had equivalent negative effects on perennial recruitment as a 85% increase in nitrogen loading. Nutrient enrichment increased algal nitrogen content and decreased tissue C: N ratios in spring and summer but not in fall. Carbon storage and nitrogen retention, measured as C and N retained in plant biomass at the end of the growth season, were increased by grazers (C: 39%, N: 24%) but decreased with increasing nitrogen loading (C: -71%, N: -74%). Our results emphasize the important role of grazers in buffering moderate eutrophication effects and illustrate how food web interactions and shifts in species composition are tightly linked to coastal ecosystem functio

Die Meeresfischerei der Zukunft

Der Klimawandel wird alle Aspekte des Lebens im Meer beeinflussen, von grundlegenden biologischen Prozessen, wie dem Überleben und Wachstum einzelner Lebewesen, bis zur Verbreitung und Häufigkeit von Arten und der Organisation mariner Nahrungsnetze. Diese Veränderungen beeinflussen die Struktur und Funktion des Ökosystems Meer und damit auch die Fischbestände und Fischereien. Viele Fischer müssen sich anpassen, wo und was sie fischen und wie reichhaltig, groß und wertvoll die gefischten Arten und der Gesamtfang sind. Viele Arten werden ihre Verbreitung polwärts verschieben, in größere Tiefen oder weiter in den offenen Ozean. Einige polare und tropische Arten werden möglicherweise aussterben. All dies wird zu einer Umstrukturierung der Lebensgemeinschaften führen mit schwer vorhersagbaren Folgen. Basierend auf verfügbaren Klimaprojektionen wird erwartet, dass die globale Fischproduktion und Fangmenge zurückgehen wird, allerdings mit großen regionalen Unterschieden, wie der Zunahme in hohen und Abnahme in niedrigen Breiten. Ein Großteil künftiger Veränderungen wird allerdings von der Entwicklung der Fischerei selbst und anderer menschlicher Einflüsse abhängen. Eine Reduzierung des Fischereidrucks sowie nachhaltiges Fischerei- und Ozeanmanagement werden nötig sein, um Fisch und Fischerei in Zukunft zu erhalten. Dabei kann der Schutz der biologischen Vielfalt im Meer als Versicherung in Zeiten des Wandels dienen

Variable and complemtary effects of herbivores on different life stages of bloom-forming macroalgae

Increased nutrient loading favors macroalgal blooms in eutrophied coastal ecosystems. The main counteracting factor on this bottom-up support is top-down control by consumers. We asked (1) whether herbivore control on 2 bloom-forming macroalgae in the Baltic Sea varies between different algal life stages, (2) whether herbivores selectively feed on Enteromorpha spp. (Chlorophyceae) thereby supporting dominance of Pilayella littoralis (Phaeophyceae), and (3) whether various herbivore species differ in their effects. In comparative field and laboratory experiments, we analyzed herbivore pressure and selectivity on germling density and adult thalli of Enteromorpha spp. and P. littoralis. In the field, herbivores reduced macroalgal recruitment by 80% within 14 d indicating strong herbivore control at early life stages. Recruits of Enteromorpha spp. were significantly preferred over P. littoralis Adult thalli of both algae showed similar growth rates, but grazing rates were significantly higher on Enteromorpha spp. In laboratory experiments, Idotea chelipes (Isopoda), Littorina saxatilis (Gastropoda) and Gammarus locusta (Amphipoda) strongly reduced germling density, whereas effects of L. littorea were weak. I. chelipes and L. saxatilis significantly preferred germlings of Enteromorpha spp. over P. littoralis. I. chelipes had strong effects on adult Enteromorpha spp. but not on P. littoralis. The effects of G. locusta and L. littorea on both adult algae were smaller and not selective, and L. saxatilis did not feed on adults at all. Different herbivore feeding modes are discussed. We conclude that strong and selective herbivory on juvenile and adult Enteromorpha spp. favors dominance of P. littoralis in the Baltic. An effective control of macroalgal blooms in eutrophied areas can be supported by sustaining high herbivore density and species richness considering the variable and complementary effects of different herbivore species on different algal life stages

Marine diversity shift linked to interactions among grazers, nutrients and propagule banks

Diverse coastal seaweed communities dominated by perennial fucoids become replaced by species-poor turfs of annual algae throughout the Baltic Sea. A large-scale field survey and factorial field experiments indicated that grazers maintain the fucoid community through selective consumption of annual algae. Interactive effects between grazers and dormant propagules of annual algae, stored in a 'marine seed bank', determine the response of this system to anthropogenic nutrient loading. Nutrients override grazer control and accelerate the loss of algal diversity in the presence but not in the absence of a propagule bank. This implies a novel role of propagule banks for community regulation and ecosystem response to marine eutrophication