Wouter Suykerbuyk

Toxic effects of increased sediment nutrient and organic matter loading on the seagrass zostera noltii

As a result of anthropogenic disturbances and natural stressors, seagrass beds are often patchy and heterogeneous. The effects of high loads of nutrients and organic matter in patch development and expansion in heterogeneous seagrass beds have, however, poorly been studied. We experimentally assessed the in situ effects of sediment quality on seagrass (Zostera noltii) patch dynamics by studying patch (0.35 m diameter) development and expansion for 4 sediment treatments: control, nutrient addition (NPK), organic matter addition (OM) and a combination (NPK+OM). OM addition strongly increased porewater sulfide concentrations whereas NPK increased porewater ammonium, nitrate and phosphate concentrations. As high nitrate concentrations suppressed sulfide production in NPK+OM, this treatment was biogeochemically comparable to NPK. Sulfide and ammonium concentrations differed within treatments, but over a 77 days period, seagrass patch survival and expansion were impaired by all additions compared to the control treatment. Expansion decreased at porewater ammonium concentrations >2,000 μmol L(-1). Mother patch biomass was not affected by high porewater ammonium concentrations as a result of its detoxification by higher seagrass densities. Sulfide concentrations >1,000 μmol L(-1) were toxic to both patch expansion and mother patch. We conclude that patch survival and expansion are constrained at high loads of nutrients or organic matter as a result of porewater ammonium or sulfide toxicity.

Seagrasses are negatively affected by organic matter loading and Arenicola marina activity in a laboratory experiment.

When two ecosystem engineers share the same natural environment, the outcome of their interaction will be unclear if they have contrasting habitat-modifying effects (e.g., sediment stabilization vs. sediment destabilization). The outcome of the interaction may depend on local environmental conditions such as season or sediment type, which may affect the extent and type of habitat modification by the ecosystem engineers involved. We mechanistically studied the interaction between the sediment-stabilizing seagrass Zostera noltii and the bioturbating and sediment-destabilizing lugworm Arenicola marina, which sometimes co-occur for prolonged periods. We investigated (1) if the negative sediment destabilization effect of A. marina on Z. noltii might be counteracted by positive biogeochemical effects of bioirrigation (burrow flushing) by A. marina in sulfide-rich sediments, and (2) if previously observed nutrient release by A. marina bioirrigation could affect seagrasses. We tested the individual and combined effects of A. marina presence and high porewater sulfide concentrations (induced by organic matter addition) on seagrass biomass in a full factorial lab experiment. Contrary to our expectations, we did not find an effect of A.marina on porewater sulfide concentrations. A. marina activities affected the seagrass physically as well as by pumping nutrients, mainly ammonium and phosphate, from the porewater to the surface water, which promoted epiphyte growth on seagrass leaves in our experimental set-up. We conclude that A. marina bioirrigation did not alleviate sulfide stress to seagrasses. Instead, we found synergistic negative effects of the presence of A. marina and high sediment sulfide levels on seagrass biomass.

Unpredictability in seagrass restoration: analysing the role of positive feedback and environmental stress on Zostera noltii transplants

1. Restoration of key species in dynamic coastal ecosystems benefits from reduction in environmentalstress. This can be realized by promoting positive feedback (intrinsic processes) orby reducing extrinsic negative forcing.2. In a seagrass (Zostera noltii) restoration project in the south-western Netherlands, weinvestigated transplantation success in relation to intrinsic processes (i.e. comparing sods vs.single shoots, transplant size, transplant configuration and transplant density) and extrinsicforcing (i.e. bioturbation by Arenicola marina, desiccation and exposure to water dynamics).In total, 2600 m2 of seagrass sods were mechanically transplanted to six intertidal flats overthe course of 5 years.3. In total, 43% of sod transplants (2?25 m2) survived at the long term, whereas single shoottransplants failed within the first 3 months. The use of larger, or more compact (sod), transplantconfigurations had no long-term effect on survival, and initial densities did not affecttransplantation success either. Reducing desiccation stress increased the transplantation successduring the first growing season. Shielding transplants from bioturbating lugworms had apositive effect on long-term survival.4. Seagrass abundance in summer was related to spring abundance, whereas winter survivalwas not related to prior seagrass abundance. At four of the six intertidal flats, transplantsgradually decreased in size over time. At the other two, extensive colonization occurredaround the transplant areas in some years and was still partly present in 2015. A correlationwith the studied environmental parameters was not found.5. Synthesis and applications. Intrinsic processes favour transplantation development duringthe growing season, allowing positive feedback. Extrinsic processes favour the development ata longer time-scale (i.e. reduction in bioturbation, thus breaking the positive feedback of thebare state). Most surprisingly, the starting colonization of two out of six tidal flats could not berelated to environmental factors (hydrodynamics, light, emergence time, sediment characteristics,macro-algae and grazing). Environmental managers can improve transplantation successby restoring the positive feedback, reducing stress, but also via risk spreading by performingtransplants over wider areas. They thereby accept the complexity of processes and unpredictabletemporal and spatial variation in which transplantation sites turn out to be successful.