SEAGRASS SYSTEMS Stability of Seagrass Systems Against Anthropogenic Impacts
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SEAGRASS SYSTEMS Stability of seagrass systems against anthropogenic impacts Dissertation zur Erlangung des Doktorgrades der Mathematisch- Naturwissenschaftlichen Fakultät der Christian- Albrechts- Universität zu Kiel Vorgelegt von Britta Munkes Kiel, 2005 Aus dem Leibnitz- Institut für Meereswissenschaften an der Christian- Albrechts- Universität zu Kiel Gutachter der Dissertation: Professor Sommer Professorin Schiel Inhaltsverzeichnis Summary ..............................................................................................................................................2 Zusammenfassung.............................................................................................................................................4 Publikationsliste ..............................................................................................................................................7 Begriffserläuterung: Definitionen und Abkürzungen ..........................................................................................8 Übersicht ..............................................................................................................................................9 Zielsetzung und Gliederung der Arbeit ............................................................................................................12 Das Untersuchungsgebiet................................................................................................................................16 Manuskript I: Zostera marina in the Greifswalder Bodden, Baltic Sea: eelgrass growth and productivity near the limit ....................................................................................................19 Manuskript II: Eelgrass - epiphyte dynamics: nutrient enrichment and grazing in an eutrophic estuary ................................................................................................................................33 Manuskript III: Impact of macroalgae canopies on growth and survival of seagrass in an eutrophic estuary of the Baltic Sea.....................................................................................47 Manuskript IV: Eutrophication, phase shift, the delay and the potential return in the Greifswalder Bodden, Baltic Sea .............................................................................................................67 Gesamtdiskussion und Schlußfolgerung .........................................................................................................83 Literaturverzeichnis ..........................................................................................................................................87 Appendix – zugehörige Veröffentlichungen .....................................................................................................99 Manuskript V: Seasonal variation of fish community and habitat structure in an eelgrass bed (Zostera marina L.) in the southern Baltic Sea ...................................................................99 Manuskript VI: Impact of macroalgae canopies on growth and survival of eelgrass in an eutrophic estuary. .............................................................................................................................103 Danksagung ..........................................................................................................................................106 1 Summary Summary filamentous algae) were estimated (manuscript II). It was assumed, that high nutrient concentrations in the Greifswalder The study presented here examines which Bodden would promote epiphytes growth, factors directly or indirectly connected to resulting in their high biomass. Surprisingly, eutrophication, are relevant to the seagrass shoots showed only low load of deterioration of seagrass populations in the epiphytes, despite high nutrient supply. As Baltic Sea. More particularly it is a study of grazing pressure, due to the very low the interacting effects of seagrass autotrophic abundances of herbivores, could be neglec- competitors (epiphytic algae, drifting macro- ted as a determining factor, it appeared that algae and phytoplankton) on seagrass. epiphytes were light limited. In contrast to the These are interesting yet largely studied and findings of many other studies of the Baltic were, in this study, investigated through the Sea and in the world, ephemer epiphytes field observations and experiments of a seem to play just a minor role in the decline seagrass population in the Greifswalder of the seagrass populations in the Greifs- Bodden, Germany. walder Bodden. This shallow water system measures over 2 500 km , but is only 5-6 m deep and is a On the basis of enclosure and exclosure field naturally nutrients sink. Over decades, this experiments, the impact of floating macro- environment has been enriched with algal mats, which, accumulate at the bottom anthropogenic overfertilizing, making it an of the seagrass meadow, were estimated area of sufficient nutrient concentrations for (manuscript III). The increasing number of plant growth year-round, resulting in strong reports about the loss of seagrass popula- phytoplankton blooms and the presence of tions, which are associated with a spread of floating algal mats. However, the remixing macroalgal mats, suggest that the decline in and resuspension of organic material limits seagrass meadows is due to the appearance light availability and plant growth in these of macroalgae. The experiments were inten- eutrophic waters. ded to provide evidence of the suggested This study reveals that shoot densities and negative impacts of the macroalgal mats on above-ground biomass of Zostera marina Zostera marina, and to estimate the strength were very low, in comparison to the values and the mechanism. The experiments show for other sites on the Baltic Sea which were 3 that macroalgal mats had a strong negative to 4 fold higher (manuscript I). effect on shoot density, biomass and growth As an adaptation to the low light supply in the of seagrass plants. Contrary to the expec- Bodden, seagrass produced relatively long tations, no high ammonia concentrations leaves. It is well known, that the most were found during the decay of macroalgae important factor determining the growth of and thus toxic effects on rhizome and seagrass is light. Also, in this study, the 78% eelgrass shoots could be disregarded. The of the variance of the eelgrass biomass and main impact of algal mats on Zostera marina leaf area can be explained by light availability was a strong light attenuation and shading of and water temperature. new shoots. During the period of macroalgal mats presence, the development of new To determine impact of ephemer epiphytes shoots was stopped. Depending on its on the availability of light supply for Zostera duration and extent drifting algae can prevent marina, seasonal development and species the regeneration of seagrass meadows and diversity of epiphytes (mostly diatoms and 2 Summary consequently result in a loss of seagrass communities. Another coauthored study dealt with the The previous manuscripts (I-III) showed that impact of predatory fishes on herbivorous light availability is the dominant factor grazers in the eelgrass community controlling the growth and survival of (manuscript V). Abundance and diversity of seagrass populations in the Greifswalder fishes were estimated by a newly developed Bodden. Irradiance is especially determined catch cage developed with the intention of by the high phytoplankton- and resuspended avoiding the usual damage on seagrass particle concentrations in the Bodden. To get shoots. Ten fish species were found, which a better understanding of the current state of showed a characteristically seasonal distribu- the seagrass meadows and the light tion. Biomass and abundance of small fishes conditions in the Bodden, the historical were correlated to water temperature and development of the macrophyte populations seagrass biomass. Stomach analyses of under the influence of the eutrophication and fishes showed, that invertebrate herbivores the changing light climate were analyzed that graze on seagrass play only a minor role over the last 25 years (manuscript IV). Due to in the diet of small fishes in the seagrass the high nutrient loading in the sixties, ecosystem in comparison to phytoplankton. phytoplankton and suspended solids These results show the observations made in concentrations increased strongly, resulting the other manuscripts, that this benthic in a clear decrease in light penetration. The ecosystem is strongly determined and modi- resulting increased light attenuation led to an fied by pelagic impacts. “ecosystem shift”, from a macrophyte dominated system to a phytoplankton In conclusion, seagrass meadows in the dominated system. Within few decades, the Greifswalder Bodden are strongly controlled predominant part of the macrophyte stock by abiotic factors (“bottom-up”), especially disappeared, promoting further light light, temperature and nutrient supply. attenuation. Despite strong reduction in Biological factors like grazing by herbivores nutrient loading after 1985, water quality did or predation (“top-down” control) just play a not improve significantly and macrophytes minor role in this eutrophic system. A stock were not able to recover (until this day). possible recovery of the seagrass meadows The resilience of the Greifswalder Bodden as may result in a higher abundance and a system dominated by phytoplankton was diversity