Biotic interactions within sandy beach ecosystems, with implications for an ecologically-sound beach nourishment = Biotische interacties in zandstrandecosystemen, met zijn implicaties voor ecologisch onderbouwde strandopspuitingen
Van Tomme, J. (2013). Biotic interactions within sandy beach ecosystems, with implications for an ecologically-sound beach nourishment = Biotische interacties in zandstrandecosystemen, met zijn implicaties voor ecologisch onderbouwde strandopspuitingen. PhD Thesis. Universiteit Gent: Gent. ISBN 978-90-902738-3-9. XX, 176 pp.
Sandy beaches are the largest coastal ecosystem on earth, covering 70% of all continental margins. As more people interact directly with beaches than with any other type of shoreline worldwide, beaches are of huge social and cultural importance. Sandy beaches have a multitude of ecological but also economic functions: they are important nursery areas for a variety of marine species and function as natural coastal defence. Beaches are also highly valuable as socio-economic areas since they are key components of many tourist destinations and are important for coastal fisheries. These activities are causing a direct anthropogenic impact and put, together with natural impacts such as sea level rise and beach erosion, a severe pressure on the sandy beach ecosystem. To preserve beaches and their important ecosystem functions, management and conservation have become critical issues, especially in the light of burgeoning global population growth, demographic shifts towards the coast, and economic prosperity. However, to develop a valuable management strategy, sound knowledge of all the aspects of the beach ecosystem is important. As studies on sandy beaches are however poorly represented in scientific literature, there are some critical gaps in basic ecological information. Although patterns on sandy beaches are well-studied, the functional beach ecosystem is largely unknown. Food web dynamics, species interactions and energetic linkages on sandy beaches are barely studied and ecosystem-wide processes as nutrient cycling, cross-system nutrient ?uxes, productivity and connectivity among metapopulations on different sandy beaches are undescribed.An overview of the current knowledge on the sandy beach ecosystem is presented in chapter 1. Both the physical characteristics and biological features of sandy beaches are reviewed. In addition, important beach functions and threats are discussed and an overview of current beach management is given.In chapter 2, a modelling approach is applied to examine the role of abiotic and biotic factors in clarifying the distribution and zonation patterns of sandy beach macrobenthos in Western Europe. The seven most important and abundant species were selected, including the amphipods Bathyporeia pilosa and B. sarsi, the isopods Eurydice pulchra and E. affinis and the polychaetes Scolelepis squamata, Nephtys cirrosa and Eteone longa. Species-specific regression models were developed, taking into account abiotic and biotic factors as explanatory variables and the selected species’ distribution as response variable. The variance in the model could in part be explained by the abiotic factors and in part by the biotic factors. The two abiotic variables used (median grain size of the sediment and emersion time) did not exclusively explain the variance in species distribution and biotic interactions were suggested to explain up to one third of the variance in species distribution accounted for by the model. Both predator-prey interactions and competition were suggested as likely biotic interactions. Although the modelling approach could not unravel the true processes and implications of biotic interactions, this study gave indications for the importance of biotic interactions on sandy beaches. Moreover, it allowed us to select the most appropriate potential cause-effect relationships to be tackled subsequently.The first hypothesis that was experimentally tested was competition between the congeneric amphipods Bathyporeia pilosa and B. sarsi (chapter 3). These two amphipods co-occur on West European sandy beaches, are morphologically very similar but show a segregated zonation pattern on the beach. A mesocosm experiment was set up to elucidate the role of competition in explaining this zonation pattern. Nine treatments, combining several densities of the two species multiplied with three food level treatments were combined and replicated five times. After a period of three weeks, the experiment was finished and the population parameters (mortality, recruitment, injuries) of the two amphipod species were determined. Results show that the two amphipods attacked each other by biting off appendages. This encounter competition was more pronounced when food was scarce and densities were high. Intraspecific competition could be significantly shown in the B. sarsi-population, while intraspecific competition could not be observed in the B. pilosa-population. Interspecific competition between both amphipods could not be demonstrated significantly based on the results of this study. Conclusively, our observations of encounter competition in B. sarsi, especially under lowered food conditions, suggest that intraspecific competition contributes to this species’ upper distribution limit and peak density in the mid-intertidal zone. As no indications of competition effects in B. pilosa populations were detected, we suggest that the high abundance of this species in the high-intertidal zone is independent from B. sarsi occurrence lower on the beach and primarily relates to lower predation pressure by epi- and hyperbenthic organisms in the high-intertidal zone, as was further studied in the following chapter.Hyperbenthic and epibenthic predators are known to be of great structuring importance for the communities on soft-bottom intertidal sediments but the great majority of the studies concentrated on tidal flats. On sandy beaches, the importance of predation by epi- and hyperbenthic predators and the trophic relationships between these predators and the macrobenthos community are far less studied and information on predation pressure is lacking. Therefore, the value of top-down regulation on the macrobenthic community of intertidal sandy beaches by brown shrimp and juvenile flatfish predators is studied in chapter 4. Two mesocosm experiments were carried out to answer general questions on predation pressure and prey selectivity. The results confirmed the role of Crangon crangon as an opportunistic omnivore on dissipative intertidal sandy beaches, similar as in other intertidal habitats. The consumption results of juvenile flatfish acknowledged S. squamata as being the most important prey for this predator group. Bathyporeia pilosa also was a substantial prey in the predator’s diet and this amphipod was significantly preferred over B. sarsi. Prey selectivity between both amphipods was assumed to be specifically based on the general size of B. pilosa and B. sarsi. Bathyporeia sarsi is larger than B. pilosa and this may be an adaptation against predation by intertidal predators of intermediate size. Furthermore, the estimated predation pressure of shrimp and juvenile flatfish was found to be extensive and the combination of substantial predation pressure with significant prey selectivity suggests that predation may be an important structuring factor for macrobenthos communities on sandy beaches.An important abiotic habitat characteristic for macrobenthos on sandy beaches is the beach sediment, as infaunal organisms live in close relation with this sediment and greatly rely on it for their food and survival. In the light of current beach nourishments, we conducted mesocosm sediment selection experiments (chapter 5) with four dominant macrobenthic species, both in allotopic and in syntopic conditions. These experiments indicated the sediment preference of these dominant species and could therefore contribute to the ecological adjustment of beach nourishments. Furthermore, the experiments in syntopic conditions showed whether interactions (competition and predation) changed the sediment preferences. As such, the latter experiments could show if species interactions are important structuring factors on the beach. The frequency distribution of sediment choices was tested with a log-likelihood test (G-test) against the null hypothesis that choices were equally distributed among the four sediment types presented. Results indicated that B. pilosa and E. pulchra preferred the finest sediment, while B. sarsi had a broader preference and also occurred in medium-coarse sediments. The polychaete S. squamata had the broadest preference and even showed a high occurrence in very coarse sediments that are not naturally occurring on sandy beaches where the animals were caught. While the preferences of the amphipods were supported by other studies, the preference of E. pulchra for fine sediments did not correspond to the results in former studies and this contrast deserves further study. The obtained preference for the polychaete was not surprising as S. squamata is a cosmopolitan polychaete, occurring both on fine-grained as well as coarse-grained beaches. These preferences imply that beach nourishment with coarse sediment will have a negative effect on B. pilosa; effects of coarser sediments on S. squamata will be rather positive. Finally, interspecific competition with the syntopically occurring amphipod B. sarsi was found to change the sediment selection of the amphipod B. pilosa towards the coarser sediments where B. sarsi occurred in lower frequencies.In chapter 6, a combined envelope-mechanistic food web model is developed, predicting the response of the beach ecosystem on beach nourishment, with emphasis on the impact of several scenarios of beach nourishment on the dominant macrobenthic species (cf. previous chapters) as well. The model consists of three major modules, one determining the abiotic conditions of the beach, a second modelling expected changes in densities and biomass of lower trophic levels (microphytobenthos and macrobenthos) and a third one predicting the maximal abundance of the most important species from higher trophic levels. Three abiotic variables determine the abundances and densities of microphytobenthos and macrobenthos along soft-sanded beaches: median grain size, total organic matter and elevation relative to the lowest tide. The model is stochastic with parameters for species envelopes and beach characteristics estimated from prior statistical distributions. The input data for these envelope models were derived from 23 beaches sampled in the period 1997-2011 along the Belgian coast. The obtained regression coefficients are used to estimate species abundances according to implemented beach characteristics in the main simulation model. In this simulation model, abundances of higher trophic levels including birds and flatfish are estimated based on their relationships with macrobenthos. The simulation model was validated by sampling two beaches from which densities of the dominant species, total AFDW and species richness were subsequently compared with simulated data according to the sample location. In general, observed species densities and total biomass matched the expected values. To illustrate the ecological value of this beach nourishment simulation model, different scenarios were tested. All model simulations indicate that the used nourishment sediment is the dominant factor in determining the effect on the ecosystem, with deterioration of the beach ecosystem after nourishment with too coarse sediment (e.g. >> than 300 µm). Based on these results, a gradient of sediment grain size could be recommended for nourishment of natural, fine-grained beaches: 200-300 µm and generally, it is advised to use sediment that resembles natural beach conditions. Furthermore, it is advised to evaluate the beach ecosystem health by a combination of different variables (biodiversity, macrobenthos biomass) since focusing on one variable can be deceptive as opportunistic species can become very abundant on a beach impacted by nourishment.In conclusion, this thesis showed that biotic interactions are present within the sandy beach ecosystem and that these interactions can have a structuring role in community patterns, as discussed more profoundly in chapter 7. In addition, the better ecosystem knowledge obtained in this study, is essential for ecologically-sound beach nourishment and sandy beach management.
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