“BOLETÍN Y ELEGÍA DE LAS MITAS” CÉSAR DÁVILA ANDRADE

Emily van Egmond obtained her PhD at the Systems Ecology section of the Factulty of Science of the Vrije Universiteit Amsterdam in 2018.

Dissertation title:

“Living on the edge: resource availability and macroinvertebrate community dynamics in relation to sand nourishment.”

PhD supervisors:

Peter van Bodegom (Leiden University) Matty Berg (Vrije Universiteit Amsterdam) Rien Aerts (Vrije Universiteit Amsterdam) Figure 1.

(top left) The polychaete worm Scolelepis squamata, which lives in the intertidal zone. Figure 2. (top right) The amphipod Talitrus saltator in a patch of wrack. Figure 3. (bottom left) Field work at the beach in autumn.

Figure 4. (bottom right) Mesh bags containing wrack were placed in drift lines on the beach, to perform our “litter bag” experiment.

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and species of macroinvertebrates present. We found large differences in the supratidal macroinvertebrate community between seasons.

Macroinvertebrates were more abundant in autumn than in spring and summer, mainly due to large numbers of fly larvae. The number of species, however, was higher in summer than in spring and autumn. Macroinvertebrate diversity varied during the seasons, with the highest diversity in summer and the lowest in autumn. The most distinct supratidal macroinvertebrate community was found in summer, while communities in spring and autumn were more similar. Young and old drift lines showed the greatest differences in diversity in spring, with more diversity in young drift lines than in old ones. Young wrack is normally swiftly colonized and consumed

by the amphipod Talitrus saltator and other

species, whereas old wrack may be less palatable or of lower nutritional quality, and the species may leave in search of other food sources. We conclude that there is both seasonal and spatial variation in the supratidal macroinvertebrate community at the Sand Motor. In particular, this field experiment stresses the importance of leaving wrack undisturbed on nourished beaches and elsewhere to support a diverse supratidal macroinvertebrate community, especially in summer.

Compared to other forms of sand nourishment, local disturbance to the macroinvertebrate community was lower at the Sand Motor, and it also provided a new, temporary habitat for other macroinvertebrates. In conclusion, the Sand Motor created a new habitat for intertidal species by increasing the spatial heterogeneity of the sandy beach. While coastal protection is the primary goal of most Dutch sandy shores, well-designed mega-nourishments also seem promising ways to support the macroinvertebrate community of the sandy beach. these conditions are present in the upper

north of the Netherlands (the Wadden Sea) and the lower south (the Zeeland Delta). Thus, the Sand Motor has locally given rise to a habitat that attracts a different intertidal macroinvertebrate community. We compared the Sand Motor beach (excluding the lagoon) with beaches subject to regular beach nourishment, to evaluate how wave-exposed intertidal macroinvertebrate communities differed across nourishment types. The number of species was higher at both the Sand Motor and regular beach nourishment than at unnourished beaches. However, abundance was lower at the Sand Motor compared to other nourishment types. This suggests that intertidal species can establish at the Sand Motor, but only in limited numbers. This could be due to non-optimal habitat characteristics or competition for food. Furthermore, the Sand Motor produced a slightly different intertidal macroinvertebrate community than beaches subject to regular beach nourishment or unnourished beaches. The Sand Motor differs from regular beach nourishments in shape, size and frequency of application. This may result in a range of environmental changes (i.e., different hydrodynamic forces, beach slope, grain size), which influence the presence of certain intertidal species and result in an altered composition of the macroinvertebrate community.

Supratidal macroinvertebrates can be found in freshly deposited wrack close to the high water line and also in older, partly decayed wrack in drift lines closer to the dunes. It is not known whether these macroinvertebrate communities differ in structure, and seasonal variation of the supratidal macroinvertebrate community has rarely been studied. To explore whether young or old wrack and season affect the supratidal macroinvertebrate community composition differently, we performed a “litter bag” experiment at the Sand Motor (Figure 5). We placed mesh bags containing wrack on the beach in young and old drift lines in the spring, summer and autumn. We collected the bags after two weeks, and determined both the number of individuals of all species encountered at the Sand

Motor. When comparing different years after completion of the Sand Motor, macroinvertebrate abundance was lower in 2014 than in other post-nourishment years, while the number of species was lower in both 2010 and 2014. The macroinvertebrate community had the least variety in 2010, and it became more varied after completion of the Sand Motor.

It is common to find large differences in the community composition between individual years at the beach. This variation may be due to the changing environmental factors on a sandy beach (e.g., average temperature, grain size distribution, food availability) or between-year differences in macroinvertebrate reproduction and survival. When comparing locations within the Sand Motor, the beach south of the sand hook had both the highest macroinvertebrate abundance and highest number of species. This may be due to the sea currents that move from south to north along this part of the Dutch coast and, hence, influence migration of intertidal species. Many intertidal macroinvertebrate species depend on currents and other hydrodynamic forces for their dispersal. As a result, the southern beach might act as a sink for migrating intertidal species, which could lead to an accumulation of macroinvertebrates. Moreover,

environmental conditions may differ from other Sand Motor locations, with different sediment characteristics and more variation in dry and wet beach.

Our most striking finding on the

macroinvertebrate community composition was that a distinct intertidal community was present in the lagoon. This community

included the amphipod Corophium volutator

and the polychaete worms Heteromastus

filiformis, Capitella capitata and Pygospio elegans, which are species commonly encountered on intertidal mudflats. In contrast to the other wave-exposed locations, the lagoon is a sheltered beach where organic matter has accumulated over the years. This kind of intertidal macroinvertebrate community is uncommon along the coast in the Netherlands. However, Figure 5.

Collecting animals after sieving sand in summer.