Quantifying biomass and carbon processing of benthic fauna in a coastal sea – past, present and future

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http://urn.fi/URN:ISBN:978-951-51-5629-7
Title: Quantifying biomass and carbon processing of benthic fauna in a coastal sea – past, present and future
Author: Ehrnsten, Eva
Contributor: University of Helsinki, Faculty of Biological and Environmental Sciences, Tvärminne Zoological Station
Doctoral Programme in Wildlife Biology
Stockholm University Baltic Sea Centre
Publisher: Helsingin yliopisto
Date: 2020-01-10
Belongs to series: Dissertationes Schola Doctoralis Scientiae Circumiectalis, Alimentariae, Biologicae - URN:ISSN:342-5431
URI: http://urn.fi/URN:ISBN:978-951-51-5629-7
http://hdl.handle.net/10138/307853
Thesis level: Doctoral dissertation (article-based)
Abstract: Benthic macrofauna is an important component linking pelagic and benthic ecosystems, especially in productive coastal seas. Through their metabolism and behaviour, benthic animals affect biogeochemical fluxes between the sediment and water column. Mechanistic models that quantify these benthic-pelagic links are crucial for understanding the functioning of coastal ecosystems and their responses to anthropogenic pressures, such as climate change and eutrophication. In this thesis the flows of carbon through functional groups of benthic macrofauna and their sediment food sources were explored using a new mechanistic model, called the Benthic Macrofauna model. The model was coupled to the hydrodynamic-biogeochemical BALTSEM model and used to simulate past, present and future biomass and metabolic carbon processing of aphotic soft-sediment communities of macrofauna in the Baltic Sea. The aims of this thesis were to identify the main drivers of macrofaunal biomass and community composition and to quantify the effects of environmental change on macrofaunal communities and their contribution to benthic carbon processing. Sedimentation of particulate organic carbon as a food source was identified as a main driver of macrofaunal biomass in two coastal areas as well as in the four largest basins of the Baltic Sea. Together with results of a food-web model of the central Baltic Sea, these results indicate that eutrophication has led to increased biomass in most parts of the ecosystem through increased productivity and sedimentation, except where counteracted by associated expanding hypoxia. Hypoxia has severe local effects on community biomass and composition, but on the scale of the Baltic Sea, biomass gains in oxic areas seem to exceed biomass losses due to hypoxia during past eutrophication. Increasing the bottom water temperature had a relatively small negative effect on community biomass in comparison to the other tested drivers, but the indirect effect of increasing surface water temperature through intensification of pelagic recycling and reduction of organic matter input to the sediment was substantial. Macrofaunal metabolism can contribute substantially to benthic carbon and nutrient processing, especially in shallow, productive coastal areas where biomass is primarily food-limited. For example, in a coastal area of the Gulf of Finland in the early 2000s, the benthic macrofauna was estimated to process up to 80% of simulated carbon input through ingestion and mineralize 40% through respiration. On the scale of the Baltic Sea, the benthic macrofauna was estimated to mineralize about 20% of organic carbon input to the sediments. These results together with a literature review suggest that the role of benthic macrofauna needs to be considered in models of coastal and global carbon and nutrient cycling. Simulations combining changes in climate and nutrient loads resulted in large reductions in benthic macrofaunal biomass and carbon processing capacity by the end of the 21st century if nutrient loads to the Baltic Sea are reduced according to the Baltic Sea Action Plan, but also if loads are kept at present levels. With increased nutrient loads, climate change counteracted the effects of increased productivity, also leading to a decrease in organic matter sedimentation, macrofaunal biomass and carbon processing capacity in the second half of the century.Bottenlevande djur utför viktiga funktioner i kustnära hav som kopplar ihop biologiska och kemiska processer i vattenkolumnen med dem i havsbotten. Bland annat bidrar de till kolets kretslopp genom att äta organiskt material som sjunker till bottnen, omvandla det i sina kroppar och avge en del som koldioxid genom sin respiration. De är också en födoresurs för många fiskarter. I denna avhandling utvecklades en mekanistisk modell för att undersöka kolflöden genom olika funktionella grupper av bottenlevande djur och deras födoresurser i sedimenten. Modellen kopplades till en hydrodynamisk-biogeokemisk modell och användes för att simulera förändringar i Östersjöns bottensamhällen historiskt och i scenarier för framtiden. Det övergripande syftet var att identifiera vilka faktorer som styr hur mycket bottenlevande djur det finns (dvs. deras biomassa), kvantifiera hur mycket kol de binder i sin biomassa och hur mycket de mineraliserar genom respiration, samt undersöka hur dessa processer påverkas av miljöförändringar såsom övergödning och klimatförändringar. Tillgången till föda i form av sedimenterande organiskt kol identifierades som den viktigaste faktor som driver biomassan. Resultaten i denna avhandling tyder på att övergödningen av Östersjön de senaste decennierna har lett till ökad biomassa av bottenlevande djur genom ökad produktion och sedimentering av organiskt material. I djupa områden har övergödningen däremot lett till syrebrist som decimerat faunan, men nettoeffekten av övergödningen verkar vara en markant ökning av den totala biomassan. Simuleringar med ökad temperatur i bottenvattnen hade en relativt liten inverkan på biomassan, medan ökad temperatur i ytvattnen ledde till ett effektivare kolkretslopp i vattenkolumnen och minskad födotillgång för bottendjuren. Simuleringarna tillsammans med en litteraturöversikt visar att bottenlevande djur har en betydande roll i kretsloppen av kol och näringsämnen i kustnära hav. På Östersjöskala uppskattades djuren mineralisera ca 20% av den årliga sedimenteringen av organiskt kol genom sin respiration, och t.o.m. 40% i ett kustområde i Finska viken. I framtiden kan minskad övergödning kombinerat med klimatförändring leda till avsevärt minskad biomassa av bottenlevande djur och därmed en svagare koppling mellan processer i vattenkolumnen och havsbottnen.
Subject: Aquatic Sciences
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