Effects of herbivorous birds on intertidal seagrass beds in the northern Wadden Sea (original) (raw)
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Effects of burial and erosion on the seagrass Zostera noltii
Journal of Experimental Marine Biology and Ecology, 2007
The effects of experimental burial and erosion on the seagrass Zostera noltii were assessed through in situ manipulation of the sediment level (−2 cm, 0 cm, + 2 cm, + 4 cm, + 8 cm and +16 cm). Shoot density, leaf and sheath length, internode length, C and N content and carbohydrates of leaves and rhizomes were examined 1, 2, 4 and 8 weeks after disturbance. Both burial and erosion resulted in the decrease of shoot density for all the sediment levels. The threshold for total shoot loss was between 4 cm and 8 cm of burial, particularly during the 2nd week. A laboratory experiment confirmed that shoots did not survive more than 2 weeks under complete burial. There was no evidence of induced flowering by burial or erosion. As well, no clear evidence was found of sediment level effects on leaf and sheath length. Longer rhizome internodes were observed as a response to both burial and erosion, suggesting a plant attempt to relocate the leaf-producing meristems closer to sediment surface or in search of new sediment avoiding the eroded area. The C content of leaves and rhizomes, as well as the non-structural carbohydrates (mainly the starch in rhizomes), decreased significantly along the experimental period, indicating the internal mobilization of carbon to meet the plant demands as a consequence of light deprivation. The significant decrease of N content in leaves, and its simultaneous increase in rhizomes, suggests the internal translocation of nitrogen from leaves to rhizomes. About 50% of the N lost by the leaves was recovered by the rhizomes. Our results indicated that Z. noltii has a high sensitivity to burial and erosion disturbance, which should be considered in the management of coastal activities.
Seagrass recovery in the Northern Wadden Sea?
Helgoland Marine Research, 2008
Aerial surveys on seagrass (Zostera spp.) indicate a three to fourfold increase in bed area from 1994 to 2006 with up to 100 km 2 or 11% of intertidal Xats in the Northfrisian Wadden Sea (coastal eastern North Sea), observed at seasonal maximum in August when Xying during low tide exposure 300 to 500 m above ground. When viewed from the air, diYculties in distinguishing between seagrass and green algae and a lack of contrast on dark-coloured mudXats are sources of error in areal estimates. Particularly the positioning of beds remote from shores was imprecise. However, the consistency in method over time gives conWdence to the inferred positive trend which is opposite to the global pattern. Both, the spatial pattern and a recent decrease in storminess suggest that sediment stability is the key factor for seagrass dynamics in this tidal area. On exposed sand Xats, high sediment mobility may be limiting and along the sheltered mainland shore land claim activities with high accretion rates may cause a scarcity of seagrass. The potential area of seagrass beds may be twice as large as the realized maximum in 2006 but eventually the rising sea level will reverse the observed seagrass expansion.
ALTERATION OF SEAGRASS SPECIES COMPOSITION AND FUNCTION OVER TWO DECADES
Ecological Monographs, 2008
Changes in the species composition and structural characteristics of marine vegetated habitats in response to climate change or local anthropogenic impacts may alter their quality as habitat for associated fish and invertebrates. Summer densities and biomass of the eelgrass, Zostera marina, declined significantly between 1985 and 2004 in Bogue Sound, North Carolina, USA, within the present-day zone of biogeographic overlap in the distribution of this subtidal temperate species and the intertidal subtropical seagrass, Halodule wrightii. Zostera decline was associated with increased spring water temperatures and water nutrient concentrations. In contrast, Halodule did not exhibit a consistent trend of temporal change. Experimental seagrass transplants indicated that Halodule has the capacity to grow at depths greater than it currently occupies, suggesting that Halodule might, over time, replace Zostera. The abundance and diversity of infaunal invertebrates were lower in seagrass beds dominated by Halodule than in those dominated by Zostera or mixtures of the two species, suggesting that changing seagrass species composition affects associated faunal assemblages. Experimental deployment of artificial seagrass patches mimicking the structure of the two species showed that both depth and structural characteristics of seagrass explain differences in faunal assemblages. Epifaunal community structure differed significantly between structurally identical seagrass mimics deployed in intertidal and subtidal habitat, and invertebrates and fishes were significantly more abundant in artificial Zostera than Halodule patches at one of the two study sites. Synthesis of these results suggests that, in the event of continued Zostera loss, with or without replacement by Halodule, important habitat functions will be lost, and secondary productivity of these lagoonal ecosystems may significantly decrease.
Marine Environmental Research, 2017
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The impact of sediment burial and erosion on seagrasses: A review
Estuarine Coastal and Shelf Science, 2008
Keywords: seagrass burial erosion sediment impacts disturbance sediment redistribution plant size a b s t r a c t The available information from experimental and descriptive studies on the effects of sediment burial and erosion on seagrasses was compiled to synthesize the information regarding the species-specific impacts and to relate them to plant characteristics. Burial thresholds (i.e. the burial levels causing 50% and 100% shoot mortality) and mortality-burial curves were estimated for the 15 seagrass species where the effects of experimental burial have been tested. All the species investigated reached 50% shoot mortality at burial levels ranging from 2 cm (Halophila ovalis) to 19.5 cm (Posidonia australis). P. australis was the most tolerant seagrass species to burial, while Thalassia testudinum was the most tolerant species to erosion. The relationships among plant size, growth, biomass and density with burial thresholds were examined. There were significant relationships between the burial thresholds and the shoot mass, the rhizome diameter, the aboveground biomass, the horizontal rhizome elongation and the leaf length of seagrass species. The leaf size and the rhizome diameter are the best predictors of the capacity of seagrasses to withstand burial. The burial thresholds estimated for seagrass species were in many cases in agreement with the burial impacts described by field observations (bioturbation), while in some cases was related to the species long-term colonization capacity (dune migration). Most human-induced impacts result in important changes of the sedimentary environment, with permanent negative effects on seagrass meadows (regression and complete destruction), whereas natural events, whether extreme (hurricane) or regular (dune migration), allow the recovery and/or adaptation of seagrasses to the burial/ erosion sediment dynamics. The extent of the effects of burial and erosion on seagrasses is speciesspecific and strongly size-dependent.
The Role of Fauna in Seagrass Habitats
2024
Biotic interactions influence ecosystems directly, through processes such as consumption and disturbance, and indirectly, by modifying environmental conditions or resources. In coastal marine systems, biotic interactions can, alongside abiotic factors, shape the growth and distribution of seagrass. Large (mega) bioturbating species, through their sediment reworking activity, are able to cause large scale impact on seagrass expansion into adjacent sand. We assessed the extent of this impact caused by mobile megabioturbators in two seagrass Cymodocea nodosa meadows of the Canary Islands, through: (1) a manipulative field experiment where access by megabioturbators to the middle and edges of seagrass beds, and adjacent sand was restricted for 4 months during the growth season; and (2) unattended video cameras where the presence and interactions of megafauna were recorded, in each of the three habitats. With the exception of herbivorous fish, which were more abundant in seagrass, other trophic groups of fish did not differ in their occupation of the three habitats (edge, middle, sand). By contrast, the results of the manipulative field experiment demonstrated a negative effect of megabioturbators on both the density and biomass of young seagrass ramets in the sand, just outside a mature seagrass meadow, but not on seagrass biomass in either edge or middle habitats of seagrass meadows. Interestingly, the exclusion of megabioturbators significantly modified the infaunal community composition, indicating the potential for indirect effects of the megabioturbators if the infaunal community composition in turn influences seagrass growth. Overall, these results indicate that megabioturbators play a direct and, possibly also indirect role in shaping the growth and patch expansion of seagrass at shallow depths. The failure of megabioturbators to modify the biomass of established seagrass also indicates an important role of selffacilitation in stabilizing seagrass.
Scientia Marina, 2017
Seagrass beds are among the most valuable ecosystems in the world but they are also among the ones most affected by human activities, and they have decreased significantly in recent decades. In many areas, such as in the Basque Country (northern Spain), seagrass beds occupy areas that are also of interest for human activities such as recreation and shellfishing. They may therefore face a number of pressures that cause damage or irreversible states. Taking into account the limited distribution of seagrass beds in the Basque Country and the interest in their conservation, an eight-month field experiment focusing on the Zostera noltei growing season was carried out to evaluate the effect of shellfish gathering. We used generalized linear models to assess different intensities of trampling and digging, as the most important pressures of shellfishing applied to Zostera noltei beds. The results indicated that shoot density of Z. noltei was negatively altered by trampling treatments and po...
Seagrass Removal Leads to Rapid Changes in Fauna and Loss of Carbon
Frontiers in Ecology and Evolution
Seagrass habitats are important natural carbon sinks, with an average of ∼14 kg C m −2 buried in their sediments. The fate of this carbon following seagrass removal or damage has major environmental implications but is poorly understood. Using a removal experiment lasting 18 months at Gazi Bay, Kenya, we investigated the impacts of seagrass loss on sediment topography, hydrodynamics, faunal community structure and carbon dynamics. Sediment pins were used to monitor surface elevation. The effects of seagrass removal on water velocity was investigated using Plaster of Paris dissolution. Sediment carbon concentration was measured at the surface and down to 50 cm. Rates of litter decay at three depths in harvested and control treatments were measured using litter bags. Drop samples, cores, and visual counts of faunal mounds and burrows were used to monitor the impact of seagrass removal on the epifaunal and infaunal communities. Whilst control plots showed sediment elevation, harvested plots were eroded (7.6 ± 0.4 and −15.8 ± 0.5 mm yr −1 respectively, mean ± 95% CI). Carbon concentration in the surface sediments was significantly reduced with a mean carbon loss of 2.21 Mg C ha −1 in the top 5 cm. Because sediment was lost from harvested plots, with a mean difference in elevation of 3 cm, an additional carbon loss of up to 2.54 Mg C ha −1 may have occurred over the 18 months. Seagrass removal had rapid and dramatic impacts on infauna and epifauna. There was a loss of diversity in harvested plots and a shift toward larger bodied, bioturbating species, with a significant increase in mounds and burrows. Buried seagrass litter decomposed significantly faster in the harvested compared with the control plots. Loss of seagrass therefore led to rapid changes in sediment dynamics and chemistry driven in part by significant alterations in the faunal community.
Journal of Sea Research
The relative impact of light, depth, hydrology, hydrodynamics, sediment and nutrients on growth of the seagrass Zostera noltei was investigated in Arcachon Bay, where the extent of seagrass beds has been decreasing for 20 years. A comprehensive multi-parameter survey of 9 sites was conducted monthly over one year. A three-parameter logistic growth model was fitted to biomass data to unravel the relative contributions of environmental factors on seagrass growth. The present study highlights the fact that, in addition to the dominant positive effect of light on plant growth, the hydrological and hydrodynamic parameters were important drivers for Z. noltei growth at the scale of the whole Bay. At the scale of individual sites, other factors such as macroalgal mats or micro-topography (such as presence of tidal pools) may also influence seagrass growth. The statistical model also suggested that the different factors contribute differently to Z. noltei growth in different sites. Finally, as the model is not able to predict biomass accurately for sites that received very little light, and seagrasses in these sites having higher chlorophyll content and longer shoots, we suspect that adaptation to low light condition was at play. In a global context of seagrass beds decline, our study provides a framework to disentangle the site-specific effects of physical and biological drivers on seagrass seasonal growth. Highlights ► Beyond light, hydrology and hydrodynamics mainly control Z. noltei growth. ► Seagrasses acclimation to low light conditions for deeper sites. ► Site-specific effects of physical drivers on seagrasses growth.