Matthijs van der Geest

A Three-Stage Symbiosis Forms the Foundation of Seagrass Ecosystems

Ancient Associations Submarine seagrass meadows are critical to fisheries and coastline protection and provide feeding grounds for many endangered species, including dugongs and turtles, and serve as a nursery for coral reef fish. The persistence and maintenance of seagrass ecosystems have been mysterious, because accumulation of organic matter in the beds should rapidly lead to toxic sulphide levels in the sediment. Using a meta-analysis and a field study, van der Heide et al. (p. 1432) atttribute the 100-million-year success of seagrasses to a three-stage symbiosis. Seagrass beds worldwide contain high densities of small lucinid bivalves that have symbiotic sulphide-oxidizing bacteria in their gills. This association appears to relieve any sulphide stress for seagrasses, while the lucinids and their symbionts profit from the accumulation of degradable organic matter and oxygen release from seagrass roots. A marine plant, small molluscs, and their resident sulfide-oxidizing bacteria survive together. Seagrasses evolved from terrestrial plants into marine foundation species around 100 million years ago. Their ecological success, however, remains a mystery because natural organic matter accumulation within the beds should result in toxic sediment sulfide levels. Using a meta-analysis, a field study, and a laboratory experiment, we reveal how an ancient three-stage symbiosis between seagrass, lucinid bivalves, and their sulfide-oxidizing gill bacteria reduces sulfide stress for seagrasses. We found that the bivalve–sulfide-oxidizer symbiosis reduced sulfide levels and enhanced seagrass production as measured in biomass. In turn, the bivalves and their endosymbionts profit from organic matter accumulation and radial oxygen release from the seagrass roots. These findings elucidate the long-term success of seagrasses in warm waters and offer new prospects for seagrass ecosystem conservation.

Seagrass–Sediment Feedback: An Exploration Using a Non-recursive Structural Equation Model

The reciprocal effects between sediment texture and seagrass density are assumed to play an important role in the dynamics and stability of intertidal–coastal ecosystems. However, this feedback relationship has been difficult to study empirically on an ecosystem scale, so that knowledge is mainly based on theoretical models and small-scale (experimental) studies. In this paper we apply a non-recursive structural equation model (SEM) to empirically investigate, at large spatial scale, the mutual dependence between seagrass (Zostera noltii) density and sediment texture, on the pristine, seagrass-dominated, intertidal mudflats of the Banc d’Arguin, Mauritania. The non-recursive SEM allows consistent estimation and testing of a direct feedback between sediment and seagrass whilst statistically controlling for the effects of nutrients and abiotic stress. The resulting model is consistent with the hypothesized negative feedback: grain size decreases with seagrass density, whereas fine grain size has a negative impact on seagrass density because it decreases pore water exchange which leads to hypoxic sediment conditions. Another finding is that seagrass density increases with sediment organic material content up to a threshold level beyond which it levels off. In combination with decreasing grain size, accumulation of organic matter creates hypoxic sediment conditions which lead to the production of toxic hydrogen sulfide which slows down seagrass growth. The negative feedback loop implies that intertidal Z. noltii modifies its own environment, thus controlling its growing conditions. To the best of our knowledge, this study is the first to demonstrate a direct negative feedback relationship in ecosystems by means of a non-recursive SEM.

Interference from adults forces young red knots to forage for longer and in dangerous places

In birds and mammals, juvenile and adult foragers are often found apart from each other. In this study, we found this is also true for red knots, Calidris canutus canutus, wintering on the intertidal flats of Banc d’Arguin, Mauritania. Not only did juveniles feed separately from adults, they also fed at places where they were more vulnerable to predation by large falcons. That the dangerous areas used by juveniles were no better feeding areas led us to reject the foodesafety trade-off that explained age-related distribution differences in many earlier studies. Instead, juveniles were displaced by adults in dyadic interactions which suggests that they suffered from interference from adults. Juveniles retreated to feeding areas that were more dangerous and yielded lower intake rates, and coped by extending foraging time by using higher, nearshore intertidal areas that were exposed for longer. When disturbed by predators in these nearshore areas, juveniles continued feeding whereas adults left. Thus, rather than compensating for increased predation danger by higher intake rates, on the Banc d’Arguin red knot juveniles foraged for longer.

How habitat-modifying organisms structure the food web of two coastal ecosystems

The diversity and structure of ecosystems has been found to depend both on trophic interactions in food webs and on other species interactions such as habitat modification and mutualism that form non-trophic interaction networks. However, quantification of the dependencies between these two main interaction networks has remained elusive. In this study, we assessed how habitat-modifying organisms affect basic food web properties by conducting in-depth empirical investigations of two ecosystems: North American temperate fringing marshes and West African tropical seagrass meadows. Results reveal that habitat-modifying species, through non-trophic facilitation rather than their trophic role, enhance species richness across multiple trophic levels, increase the number of interactions per species (link density), but decrease the realized fraction of all possible links within the food web (connectance). Compared to the trophic role of the most highly connected species, we found this non-trophic effects to be more important for species richness and of more or similar importance for link density and connectance. Our findings demonstrate that food webs can be fundamentally shaped by interactions outside the trophic network, yet intrinsic to the species participating in it. Better integration of non-trophic interactions in food web analyses may therefore strongly contribute to their explanatory and predictive capacity.

Banc d'Arguin - Food webs with trophic effects by seagrass removed

This file contains matrices from which we we removed seagrass (Zostra noltii) and any species exclusively feeding on it.