Veldverslag strandsurvey Natuurlijk Veilig : Maart-juni 2019

In de periode 23 maart tot 23 juni 2019 heeft Wageningen Marine Research in opdracht van Rijkswaterstaat in het kader van Natuurlijk Veilig 1 à 2 keer per maand een strandsurvey uitgevoerd in de ondiepe brandingszone van de Noordzee bij Katwijk aan Zee, Castricum aan Zee en Texel. De survey was opgezet om de verspreiding en abundantie van juveniele platvis in de vooroever te bepalen en gegevens te verzamelen van (a)biotische factoren die deze verspreiding bepalen. Het doel is deze gegevens te gebruiken om mogelijke effecten van zandsuppleties in de brandingszone op juveniele platvis te voorspellen. Per bemonstering zijn er per studiegebied zes transecten van elk 100 m bevist met een 1,8 m kornet (maaswijdte 1 cm halve maas) op een waterdiepte van 0,2-1,0m. Daarnaast is per bemonstering op iedere locatie een sedimentmonster genomen bij de eerste drie transecten, en zijn er gegevens verzameld over doorzicht, watertemperatuur en saliniteit. Afhankelijk van de mankracht zijn op sommige dagen en locaties enkele transecten ook bevist met een 1 m, 1,5 m of 2 m boomkor met een maaswijdte van respectievelijk 1 cm, 1 cm, en 0.5 cm (halve maas)

First Field-Based Evidence That the Seagrass-Lucinid Mutualism Can Mitigate Sulfide Stress in Seagrasses

Seagrass meadows form vital ecological components of coastal zones worldwide, but are rapidly declining. Large-scale seagrass diebacks have been related to accumulation of toxic sulfide in the sediment, a phenomenon predicted to occur more frequently in the near future due to ongoing global warming and increasing organic loading of coastal systems worldwide. Recently, a facultative mutualism between seagrasses and lucinid bivalves with endosymbiotic sulfide-consuming gill bacteria was discovered that may prevent toxic sulfide accumulation in seagrass sediments. Yet, direct field-based evidence for the importance of this mutualism in alleviating sulfide stress in seagrasses is currently lacking, as well as how its role may change when sediment sulfide levels increase due to environmental change. Here, we investigated the sulfide detoxificationfunction of this seagrass-lucinid mutualism and its resilience to organic-loading induced sulfide stress in a temperate lagoon system (Thau lagoon, France), using a correlative field survey and a full factorial field experiment. The field survey revealed a strong positive correlation between seagrass above-ground biomass and lucinid densities, and pore water sulfide concentrations close to zero at all sites. Furthermore, the field experiment revealed that addition of organic matter (starch mixed with sucrose) increased sedimentary sulfide intrusion into seagrass (Zostera noltei) leaves (a proxy for sulfide stress), while experimentally enhanced lucinid densities reduced sulfide intrusion, regardless of addition of organic matter. Moreover, addition of organic matter reduced seagrass rhizome biomass and increased pore water sulfide levels, lucinid tissue sulfur content, lucinid condition (expressed as flesh/shell dry weight ratio), and total lucinidbiomass, while enhancement of lucinid densities reduced lucinid condition. These results provide the first field-based evidence that lucinid bivalves and their sulfide-oxidizing gill symbionts mitigate sulfide stress in seagrasses, and suggests that the dependence of seagrass on this seagrass-lucinid mutualism will increase under conditions of enhanced sediment sulfide production, as predicted for the near future. Therefore, we suggest that awareness of the ecological importance of the seagrass-lucinid mutualism may be instrumental for designing new measures for improving long-term restoration success and seagrass resilience to global change

Concurrentie bevordert onderwijsrendement niet

Keuzevrijheid bestaat al jaren in het voortgezet onderwijs en maakt dat scholen onderling concurreren. Door de beschikbaarheid van meer informatie kunnen ouders en leerlingen steeds beter kiezen. Concurrentie tussen scholen leidt echter niet tot meer onderwijsrendement in termen van cijfers of slagingspercentages

School Choice and Competition

The literature suggests that competition among schools might increase quality. However, not much empirical evidence is present as only a few countries allow competition at a large scale. One exception is the Netherlands. Free parental choice is the leading principle of the Dutch education system since the beginning of the 20th century. Based on panel data for the Netherlands we show that there is a relation between competition and student achievement in upper secondary education, but that it is negative. In addition, private schools have higher quality levels

How the carrying capacity of the Wadden Sea regulates the number of wintering waders at Banc d’Arguin

Omdat veel van de in Afrika overwinterende steltlopers gebruik maken van de Waddenzee als opvetstation, zou de voedselsituatie in de Waddenzee wel eensvan invloed kunnen zijn op het aantal in Afrika overwinterende steltlopers. Om dit te onderzoeken wordt in dit artikel gekeken naar Kanoeten en hun voedsel, inde Wad denzee en op de Banc d’Arguin, vroeger en nu, daarbij gebruikmakend van de rigoureuze veranderingen die de laatste jaren in de Waddenzee hebben plaats gehad.The number of Red Knots Calidris canutus wintering at the Banc d'Arguin, Mauritania, has declined over the last two decades. The fact that food densities in this relatively undisturbed ecosystem have increased suggests that the reason for the declining Knot abundance needs to be sought elsewhere along the flyway. With the Wadden Sea acting as a major stopover site for Red Knots wintering at the Banc d'Arguin (at least in spring), we hypothesize that the declining food stocks in the Wadden Sea are responsible for the reduced number of Red Knots wintering in Mauritania. Speculating further, we bring up the idea that the declining Red Knot population has led to reduced predation pressure on the bivalve stocks at the Banc d'Arguin, which may be the reason why food densities in Mauritania have gone up. Others have brought up an alternative, but non-mutually exclusive, hypothesis explaining this increase by an increased algal production. With production rates in individual shellfish presumably being under strong density-dependent control, the reduced predation hypothesis (in contrast to the alternative hypothesis) predicts lower per capita shellfish production rates nowadays than two decades ago.</p