Restoring aquatic food webs bottom-up : Improving trophic transfer through lake restoration project Marker Wadden

Summary

Lakes are important freshwater ecosystems and although they only cover ~3% of the Earth’s surface, they provide a substantial number of ecosystem services. Many lake ecosystems worldwide are suffering from severe ecological degradation attributed to multiple anthropogenic stressors. Consequently, this leads to the loss of biodiversity and damage to ecosystem services provided by lakes. To preserve and restore biodiversity and ecosystem services supported by lakes, ecological degradation through global change needs to be counteracted. One way to achieve this is through lake restoration projects. To date, lake restoration is often based on single-stressor abatement approaches, in particular nutrient loading reduction. While this has been often shown to successfully improve water quality, it also may cause unintended declines of higher trophic production, such as fish and water birds. Besides, the desired goals of single-stressor abatement approaches could be counteracted by other factors, such as wind-induced sediment resuspension that may lead to enhanced internal nutrient loading, specifically for shallow lakes. Therefore, nature-based multiple stressor management is needed to both improve lakes’ ecological status and maintain their ecosystem services.

In this thesis, I study the effectivity of an innovative lake restoration approach which is based on such a multiple-stressor intervention strategy, the Marker Wadden project in lake Markermeer, The Netherlands. Marker Wadden is a 1000-ha man-made archipelago consisting of five islands with natural shorelines and lagoons, aiming to stimulate the development of a wind protected littoral zone that is currently largely lacking due to basalt dikes surrounding lake Markermeer (Chapter 2). The goal of Marker Wadden is to create a bird and fish paradise by stimulating the aquatic food web development bottom-up. In my thesis, my aim is to understand whether Marker Wadden will improve trophic transfer from phytoplankton to zooplankton, and thereby support higher trophic levels, thus achieving the overall goal of the project. Specifically, I tested the following overarching hypotheses:

1. Creating shelter against wind will increase trophic transfer between phytoplankton and zooplankton in shallow lakes by decreasing the suspended solids concentration;
2. Creating shelter against wind will increase trophic transfer by supporting habitat for more types of primary producers, which in turn can support a higher consumer diversity and biomass;
3. Creating littoral zones will increase nutrient availability coupled with improved light availability which increases primary producer quantity and quality, thereby stimulating the food web bottom-up and increasing trophic transfer.

To achieve this, I combined different approaches ranging from laboratory experiments and field mesocosm experiments to field monitoring.

My results showed that reducing water turbulence reduced suspended solid concentrations in the water column and facilitated zooplankton biomass build-up in an indoor microcosm experiment (Chapter 3). Similarly, providing shelter reduced suspended solid concentrations and enhanced trophic transfer efficiency between phytoplankton and zooplankton in a mesocosm field experiment (Chapter 4). These results suggest that shelter provided by Marker Wadden can enhance trophic transfer between phytoplankton and zooplankton if Marker Wadden reduces wind-induced turbulence, which I show decreases the suspended solids concentration. Reduced wind-induced turbulence may protect zooplankton from shear forces, especially the large body sized ones. Furthermore, reduced mixing may release zooplankton from feeding interference with suspended solids as this may mechanically complicate food collection or dilute gut content. In contrast, I found higher suspended solid concentrations on locations within Marker Wadden compared to those outside Marker Wadden, even though these locations that were in between the islands can be considered sheltered (Chapter 5).

In the indoor microcosm (Chapter 3) and field mesocosm (Chapter 4) experiment I also tested the effects of water turbulence and shelter, or the absence of turbulence, on the dominance of different types of primary producers. The results showed that decreased turbulence (Chapter 3) and shelter (Chapter 4) had significant effects on the interaction among different types of primary producers by changing physical, chemical, and biological conditions in the water column. These results suggest that diverse types of primary producers could colonize Marker Wadden as a gradient of shelter has been provided. Furthermore, I show that shelter also facilitated the abundance of invertebrates, such as Gastropoda (Chapter 2 and 4), which suggests that shelter may result in higher food web complexity, offering alternative pathways to stimulate higher trophic levels.

I applied the light:nutrient hypothesis in the restoration context of Marker Wadden and found that nutrient availabilities were significantly higher on sheltered locations within Marker Wadden compared to those outside of Marker Wadden (Chapter 5). These increased nutrient availabilities improved the quantity and quality (expressed as carbon:nutrient stoichiometry) of the phytoplankton, thereby supporting higher zooplankton biomass on sheltered locations inside Marker Wadden compared to outside Marker Wadden (Chapter 6). However, I did not find higher light availability in the sheltered areas of Marker Wadden, because suspended solids concentrations in the water column were higher at locations in Marker Wadden as compared to those outside of Marker Wadden. Interestingly, my findings reveal an optimum light:nutrient ratio at which phytoplankton quantity is highest, which provides a quantitative extension of the current light:nutrient hypothesis (Chapter 5). These changes in quantity and quality of primary producers (Chapter 5) may also cascade to higher trophic levels. Indeed, I demonstrated that total zooplankton biomass showed also an optimum at intermediate light:phosphorus (TP) ratios, and thereby largely followed phytoplankton biomass (Chapter 6). However, I also found that different zooplankton taxa exhibited specific optima along the light:TP gradient.

Overall, I conclude that the creation of a littoral zone by restoring land-water connections in the Marker Wadden project can be considered as a form of nature-based multiple stressor management, increasing shelter, nutrient availability and habitat heterogeneity in the form of dominance by multiple primary producers. This results in improved trophic transfer between phytoplankton-zooplankton as well as increased abundance of invertebrates as an alternative food source. Together, this may facilitate the recovery of higher trophic organisms, in particular fish and water birds. As such, Marker Wadden, as a forward-looking approach to enhancing ecological integrity while maintaining ecosystem services, can provide a new direction for future lake restoration efforts.