Macroalgal contribution to benthic macrofaunal communities and food webs in shallow coastal habitats of the Baltic Sea

• Autores: Tuomas I. Kahma
• Directores de la Tesis: Iván Franco Rodil (dir. tes.), Alf Norkko (codir. tes.)
• Lectura: En la Helsingin yliopisto = University of Helsinki ( Finlandia ) en 2022
• Idioma: español
• Número de páginas: 51
• Enlaces
  • Tesis en acceso abierto en: urn.fi
• Resumen

  • Coastal zones are vulnerable to human activities, and eutrophication is one of the most severe anthropogenic stressors of coastal ecosystems. The decline of Fucus vesiculosus in the Baltic Sea has been of major concern in the past decades. The present thesis highlights the role of this perennial macroalgae not only as a habitat-forming key species maintaining biodiversity in rocky seafloor communities but also as a cross-habitat provider of energy subsidy to adjacent softsediment ecosystems and associated macrofauna communities.

    In coastal ecosystems, the flow of detrital matter connects spatially distinct habitats, and the provision of such cross-habitat energy subsidies is fundamental for many ecological processes. The local primary production and carbon export potential of large macrophytes can be very high.

    However, the overall importance of macrophyte-originated detritus for coastal food webs is not fully understood. In oceanic systems, the role of large kelps as an organic matter subsidy has been studied previously. There is growing evidence that macroalgae-derived detritus are significant spatial subsidies from shallow rocky vegetated habitats to nearby soft-sediment shallow areas and to deeper offshore areas, where it fuels benthic macrofauna food webs. In shallow habitats of the Baltic Sea, habitat-forming rockweed species Fucus vesiculosus forms abundant and highly productive underwater forests, which host a high diversity of associated macrofauna. The carbon export potential of F. vesiculosus to nearby shallow bare sediment habitats has been recently recognized, but we know little about the role of this perennial macroalgal species in the coastal food webs and the dynamics of associated benthic macrofauna communities.

    The general aim of this thesis was to examine the dietary contribution of macrophyte-originated detrital matter, especially drifting F. vesiculosus, to the associated benthic macrofauna communities in coastal areas of the Baltic Sea influenced by large external inputs of macrophyte detritus.

    Additional aims were to investigate how this role may change spatially and seasonally and to provide insights into the fate of primary production in coastal systems dominated by large macrophytes and affected by increasing ephemeral algal accumulations. A stable isotope approach and Bayesian modelling were applied to assess the contributions of macrophyte-originated components to the adjacent basal food webs potentially influenced by drifting detrital macrophyte matter. The study took place in Hanko Peninsula, SW Finland.

    There is a high diversity of potential macrophyte food sources in the shallow vegetated habitats of the Baltic Sea since both marine and limnic species are present. The results indicated that the dominating habitat-forming macrovegetation of a habitat (F. vesiculosus or angiosperm plants) does not necessarily affect the contribution of the macrophyte component in the diets of detritivorous benthic infauna. The stable carbon and nitrogen isotopic niche structures of a particular habitat, however, are strongly affected by the dominant macrovegetation assemblage.

    In the Storfjärden Bay, most of the sedimentary carbon on the seafloor seemed to be of autochthonous origin, despite the proximity of riverine runoff. In offshore areas, pelagic primary production dominated as the main energy source of the benthos, but the results showed that the carbon export from F. vesiculosus can have a large contribution to the adjacent soft-sediment food webs also here. Depth was an important factor regulating energy source dynamics. The results from 6 different depths (from 3.5 to 45 m) indicated that the rockweed F. vesiculosus fuels soft-sediment benthic macrofauna communities in the direct proximity of shallow vegetated habitats, but it can also have a significant contribution in deeper soft-sediment areas of the coastal ecosystem. In shallow (< 5 m) vegetated habitats, F. vesiculosus is one of the main energy sources of the clam Macoma balthica, but also obligate suspension-feeding mussels (Mytilus trossulus) can incorporate some of the fine-sized detrital matter exported from F. vesiculosus rocky bed communities. In deeper areas (> 15 m), the amount of this cross-habitat energy subsidy might depend on the magnitude of physical transport processes.

    A temporal field study, examining species-specific dietary responses across three sampling dates, showed that the preferred feeding strategy, but not the season, mainly shapes the dietary compositions of the abundant benthic macroinvertebrate consumers. The results show that accumulations of drifting filamentous algae are rapidly incorporated into the food web through epigrazers. An in situ field experiment simulating a natural accumulation event and subsequent decaying process of F. vesiculosus at different depths showed that algal decomposition progressed relatively slowly. Detectable incorporation of Fucus-derived matter to epigrazers and detritivorous bivalves occurred after a few weeks, while simultaneously the incorporation of filamentous algae decreased over time. Hence, it can be suggested that the ecological role of decomposing F.

    vesiculosus is more important in areas where the algal matter can accumulate for several months.

    The effect of depth influenced the food incorporation of typical epigrazers. As a rough quantitative estimate, the effect of depth (3 m vs. 6 m) on the proportion incorporated was approximately 10 % units between the two depths, which was roughly similar to the observed temporal dietary variations in the studied area