Ecological implications of changes in polychaetes population in a shallow Danish estuary

Matthieu Delefosse

Publikation: AfhandlingPh.d.-afhandling

Abstrakt

Burrowing polychaetes are typically the most numerous and active class among macrobenthic infauna in Danish estuaries. Their ecological role is well-recognized and has been often associated with their bioturbating activities. Construction and maintenance of galleries as well as ingestion and defecation displace sediment particles. Ventilation of their burrow irrigates the sediment. These activities transform the environment for other species: from microbes to plants (MS5).
Given the important relations between bioturbating polychaetes and their ecosystem, any significant changes in population of a key species may have consequent impact on the ecosystem. The introduction of non-native species is an important vector of change for the often species-poor benthic community of Danish estuaries. Non-native species may bring new functions but also interact with native fauna species.
The work presented in this thesis evaluates the effect of population change in size and distribution of 3 polychaete species in a shallow Danish estuary - Odense Fjord: the 2 ecologically important Arenicola marina and Nereis diversicolor and the non-native species Marenzelleria viridis. We focus on the effect of the dynamic of the polychaete population on sediment biogeochemistry and the seagrass Zostera marina.
In MS1, the results obtained from different survey indicate that M. viridis was first recorded in 2002 in this estuary and is now present in about 50% of the estuary. The loss of seagrass habitat in some areas of the estuary in late 1990’s and early 2000’s opened bare areas of sediment (5-10km2) where the large bodied A. marina is now present. We also estimate that the total population in A. marina doubled within the last 10 years. There were indications of interaction between N. diversicolor and M. viridis in areas under disturbance (hypoxia or strong bioturbation). To date N. diversicolor remains the prevailing polychaete in this estuary. In the “worst case scenario” assuming that N. diversicolor is completely replaced in areas where the non-native species can survive, there could be a 3-fold reduction in the spatial distribution of N. diversicolor but with a limited effect for its overall population in Odense Fjord.
We further investigated how the established non-native M. viridis affect sediment biogeochemistry and compared its effect with that of N. diversicolor in MS4. M. viridis increased sulfate reduction rates in the sediment and created more reduced conditions than N. diversicolor. This was probably caused by the different ventilation and irrigation mechanisms of the 2 species (MS6). M. viridis deep burrow ventilation irrigates the sediment along the wall of the blind-ended burrow which results in the percolation for an average sized M. viridis of about 0.01ml/min of water rich in nutrients and reduced compounds such as sulfide. Provided the large spread of M. viridis in this estuary, the introduction of this non native species in Odense Fjord may have important consequences for the structure of the microbial and infaunal benthic community in Odense Fjord.
We evaluated how the feeding or defecating activities of the 3 polychaetes affected seagrass Zostera marina reproductive success through seed burial (MS2). The 3 worms buried seeds either directly by grabbing the seeds (N. diversicolor) or indirectly by covering seeds with their fecal pellets. The emergence of the seedling is only possible if the seed is positioned within the top 6 cm of the sediment (MS3). We estimate that in areas where the abundance of A. marina >8 ind m-2, the eelgrass reproductive success may be compromised. The recent increase in the distribution of A. marina most probably reduced the opportunity for eelgrass to recolonize this estuary.
OriginalsprogEngelsk
Udgiver
StatusUdgivet - 2. jan. 2012

Bibliografisk note

Available at the library of the University of Southern Denmark

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