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Contains fulltext : 199582.pdf (Publisher’s version ) (Open Access) SER Europe Conference 2018E Conference 2018, 9-13 September 2018, Reykjavik Iceland.

Seagrass meadows are vital ecosystems in coastal zones worldwide, but are also under global threat. One of themajor hurdles restricting the success of seagrass conservation and restoration is our limited understanding of ecologicalfeedback mechanisms. In these ecosystems, multiple, self-reinforcing feedbacks can undermine conservation efforts bymasking environmental impacts until the decline is precipitous, or alternatively they can inhibit seagrass recovery inspite of restoration efforts. However, no clear framework yet exists for identifying or dealing with feedbacks to improvethe management of seagrass ecosystems. Here we review the causes and consequences of multiple feedbacks betweenseagrass and biotic and/or abiotic processes. We demonstrate how feedbacks have the potential to impose or reinforceregimes of either seagrass dominance or unvegetated substrate, and how the strength and importance of these feedbacksvary across environmental gradients. Although a myriad of feedbacks have now been identified, the co-occurrence andlikely interaction among feedbacks has largely been overlooked to date due to difficulties in analysis and detection.Here we take a fundamental step forward by modelling the interactions among two distinct above- and belowgroundfeedbacks to demonstrate that interacting feedbacks are likely to be important for ecosystem resilience. On this basis, wepropose a five-step adaptive management plan to address feedback dynamics for effective conservation and restorationstrategies. The management plan provides guidance to aid in the identification and prioritisation of likely feedbacks indifferent seagrass ecosystems

Contains fulltext : 161856.pdf (Publisher’s version ) (Closed access)

Contains fulltext : 151373.pdf (Publisher’s version ) (Open Access) 38 p.

1. Populations of marine megaherbivores including green turtle (Chelonia mydas) have declined dramatically at a global scale as a result of overharvesting and habitat loss. This decline can be expected to also affect the tolerance of seagrass systems to coastal eutrophication. Until now, however, simultaneous effects of topdown control by megaherbivore grazing and bottomup control by nutrient input have not been tested experimentally.2. We therefore investigated the interacting effects of nutrient (N and P) addition and mimicked green turtle grazing on seagrass and epiphyte productivity, seagrass biomass and nutrient contents in exclosures at a pristine seagrass site in the Indo-Pacific region (Kalimantan, Indonesia).3. Grazing almost doubled leaf biomass production rates, while nutrient addition (N+P, slow-release granules) did not have an effect on these rates. Rhizome biomass was, however, strongly reduced by nutrient addition. In contrast to phosphorus, tissue nitrogen contents increased after nutrient addition, showing that nitrogen was not limiting primary productivity. Epiphyte growth was, however, strongly correlated with high water column P concentrations, indicating an indirect negative effect of eutrophication when turtle grazing would be absent. We calculated that green turtle leaf grazing leads to substantial exports of N and P, at rates of at least 8% of the standing stock per day equalling the daily seagrass production, up to 13 (N) and 1.4 (P) mg m(-2) day(-1).4. Synthesis. By combining our quantified effects with literature data, we propose a conceptual model of seagrass functioning under megaherbivore leaf grazing and eutrophication. In tropical seagrass systems with high green turtle grazing pressure, grazing alleviates the negative effects of eutrophication by the stimulation of seagrass production and concomitant nutrient uptake, the increased export of nutrients and the indirect prevention of low below-ground biomass. Similar to the role of terrestrial megaherbivores, these strong top-down controls show the pivotal role of green turtles in current coastal systems, which is lacking in systems where their numbers have greatly declined. These marine megaherbivores do not only drive structure and functioning of their foraging grounds but also increase the tolerance of seagrass ecosystems to eutrophication.

Contains fulltext : 225923.pdf (Publisher’s version ) (Open Access) Radboud University, 19 november 2020 Promotores : Lamers, L.P.M., Heide, Tijsse van der, Bouma, T.J. Co-promotor : Lengkeek, W. 203 p.