The research project “Nature in Production” on the Marker Wadden is a joint effort of Natuurmonumenten, the Netherlands Institute of Ecology (NIOO-KNAW), the Radboud University and the University of Groningen. The program started in 2017 and is funded by the Gieskes-Strijbis Fund. The team consists of three PhD students and one postdoc, that each focuses on a different aspect of the rapidly developing nature on the islands.

Marloes van den Akker from Radboud University studies the nutrient release from the Marker Wadden sediment that may stimulate ecosystem productivity, and how biota-water-sediment interactions influence this process. Originating as marine sediment from the Zuiderzee, the sediment with which the Marker Wadden are built consist of a mixture of marine clay, sand and some peat, which is very nutrient rich, as supported by the nutrient availability found in pore water extractions. As a result, Phragmites australis and Typha angustifolia, which are sown at Marker Wadden to jump-start reed marsh formation and prevent willow encroachment, are growing rapidly. This in turn is very attractive for grazing Greylag geese, hence at present, the Marker Wadden landscape is formed by an interaction between sediment, water level, plants and geese. This interaction does not only determine the vegetation development, but also soil subsidence, greenhouse gas emissions and food web assembly, which Marloes follows using exclosures and paired control plots.

Left: Starting from scratch: in May 2018 we planted reed in half of the exclosures and control plots on Marker Wadden. Right: in September 2018 the effect of grazing by Greylag geese was very visible: the reed grew fast, if protected from grazing.

Yvonne Kahlert from the University of Groningen studies the productivity of the islands for insects. Yvonne started by placing insect traps (malaise traps) on the Marker Wadden from mid-April through mid-August in 2018 to study the abundance of insects – inspired by the recent insect decline study using similar methods. Furthermore, she placed transects with pitfall traps to collect the ground dwelling arthropods. In total she collected almost 1000 arthropod samples during only one season. Highlights so far: the Marker Wadden are a hotspot for insect abundance, of which >90% are chironomids – so the aquatic habitat fuels terrestrial productivity. The chironomids are likely local, as also in benthic samples there are many. Multiple ground beetles of the genus Bembidion were found that are typical of riverine habitats, the small beaches, for instance Bembidion velox. Many Linphyiidae spiders were found, and there was an explosion of ladybirds. Over the season of 2018 the arthropod diversity increased and a lot of parasitic wasps were found that co-occur with their host fly species. The food web assembly progressed over the season, with particularly a high number of predators on land and a generally strong nutrient subsidy from the aquatic to the terrestrial habitat. In 2019 Yvonne continued her work with another field season, of which the first results will appear soon.

Left: Yvonne at work at one of the malaisetraps. Right: A close-up shows the huge amounts of chironomids – the non-stinging ones – that were present for most of the spring and summer.

Hui Jin from the Netherlands Institute of Ecology studies the part of Marker Wadden that is less visible: the underwater world. Around the Marker Wadden a new underwater landscape has formed, with shallow parts among the islands and sheltered parts at the lee side, depending on the wind direction. Hui is measuring the response of the phytoplankton, zooplankton and benthic communities to these different habitats by comparing the shallow sheltered sites with the open water at the different exposed sites around Marker Wadden. He found that there is a strong increase in nutrient availability in the water among the islands, increasing phytoplankton abundance, but also phytoplankton quality as food source for zooplankton when considering its carbon: phosphorous ratio. This corresponds with high abundance of zooplankton, and in the sheltered sites which were still free of fish, huge Daphnia were found. There is strong sediment resuspension in the very shallow sites, but this seems favorable for the phytoplankton as the suspended sediment provides nutrients in the water column. Therefore the base of the food web seems to profit from the Marker Wadden so far, but for now the effect seems mostly local and not to extent very far into the lake.

Left: Hui taking water samples. Right: Concentrated filtered zooplankton sample.

Casper van Leeuwen from the Netherlands Institute of Ecology studies the effects of the Marker Wadden on fish as a postdoc. For many fish species, their populations in the Markermeer have been declining over the last decades. Among possible reasons for these declining populations is the homogeneous nature of the lake: it lacks natural gradual shorelines, sheltered areas for fish to spawn and grow up, and the turbidity inhibits growth of macrophytes that can protect juvenile fish against depredation. The aim of our fish research on the Marker Wadden is to determine how and how much the islands can potentially benefit fish in the lake. During the springs and summers of 2019 and 2020 we observed adult fish spawning in the sheltered warmer waters between the islands, and found juvenile fish of many different species in locally very high densities. During the season large amounts of zooplankton become available as food for juvenile fish. In the winter of 2020 we equipped 90 fish (bream, ide, roach, pikeperch and perch) with Vemco’s transmitters, and placed 16 hydrophones within and around the archipelago that continuously try to detect the fish again for the coming year. We aim to couple adult habitat selection with juvenile recruitment and survival, food availability in the new ecosystem, and with this quantify a possible nursery function of the Marker Wadden on larger spatial scales.

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