Many of the world's wetlands may be profoundly affected by climate change over the coming decades. Although wetland managers may have little control over the causes of climate change, they can help to counteract its effects through local measures. This is because direct anthropogenic impacts, such as water extraction and nutrient loading, work in concert with climate change to damage wetlands. Control of these local stressors may therefore ameliorate undesired effects of climate change, such as a shift towards dominance by invasive floating plants, increasingly frequent cyanobacteria blooms, or extinction of key species. Using the iconic Doñana wetlands in Spain as a case study, we illustrate how the concept of creating a "safe operating space" may be implemented to better ensure that ecosystems do not surpass thresholds for collapse during an era of global change.

Within intertidal areas of European Atlantic coasts the distribution of the small seagrass Zostera noltii and the halophyte Spartina anglica can partially overlap, despite numerous biomechanical, demographic and ecophysiological differences. Both species are known to be ecosystem engineers that modify their habitat by reducing hydrodynamic energy within their canopies. In this study we investigate the influence of biomechanical (i.e. shoot flexibility) and demographic (i.e. shoot density) characteristics of these intertidal plants on their interaction with unidirectional currents to (1) understand their differences in ecosystem engineering capacity and (2) identify which physical traits explains these differences. In a flume tank, hydrodynamic variables were measured within transplanted S. anglica and Z. noltii meadows, and their corresponding simplified mimics. The results revealed that stiff canopies had a larger potential capacity (relative to flexible ones) to trap sediment, as in these vegetations velocity reduction within the canopy combined with a sufficient volumetric flow rate to provide sediment for settling. Flexible canopies were most efficient at reducing erosion by reconfiguration of their leaves. Shoot density increased the magnitude of these effects when values were moderate. However, the capacity to increase sediment accretion disappeared when the maximum velocity attenuation was reached and the flow of water was relocated on top of the canopy. These habitat modifications may provide ecological benefits for saltmarsh and seagrass species. For saltmarsh plants, the rigid shoots allow lateral expansion of their populations via increased sedimentation. For seagrasses, the dense and flexible shoots typical of temperate intertidal populations provide efficient protection from erosive forces, while at the same time helping to avoid stresses, such as drag forces and high sedimentation rates.

The hypotheses that (1) different seagrass morphologies may facilitate different nutrient uptake rates under similar hydrodynamic forcing and (2) this effect on nutrient uptake rates is spatially explicit, with the highest uptake rates at edges of patches, where currents and turbulence are highest, were examined under unidirectional flow conditions. Homogeneous patches (2 m long) of two seagrass species (Cymodocea nodosa and Zostera noltii) with contrasting shoot size and density were placed in a race track flume. 15 NH z 4 uptake and hydrodynamic properties along a gradient from outside to inside the patch were measured at a range of current velocities (0.05 to 0.3 m s 21 ). For each velocity we also determined the height and bending of the canopy. Water velocity affected the ammonium uptake rate of both species. The almost double uptake rates of C. nodosa shoots, compared to those of Z. noltii, were mainly attributed to a twofold difference in the within-canopy water flow (Q c , m 3 s 21 ). Spatial patterns in canopy water flow were highly correlated with spatial patterns in NH z 4 uptake, thereby explaining the 20% higher uptake rates at the leading edge of both canopies. The correlation between spatial patterns in canopy water flow and ammonium uptake rates underlines the role of canopy and patch configuration in determining the functioning of seagrass landscapes and their associated ecosystem services, such as nitrogen assimilation.

In this paper we investigate the temperature sensitivity of the photosynthetic process of the benthic diatom Cylindrotheca closterium grown in light-limited turbidostat cultures at two different growth rates. Photosynthesis was measured as the rate of oxygen evolution and as the photosystem II (PSII) electron transport rate (ETR). The photosynthetic efficiency (a), as measured by both methods. was rather insensitive to temperature, and decreased significantly only at the extreme temperatures used (S and 3S'C). The maxImum PSII quantum efficiency (Fv/F m ) showed a small but significant trend of reduction with increasing temperature. However, the maximum rate of photosynthesis (pBmax and ETR max ) was extremely temperature sensitive. The effect of temperature on the relationship between pB and ETR was limited to the most extreme temperatures investigated; deviations from linearity were most extreme at SoC and different conversion factors were observed at Sand 3YC. A short-term change in temperature (lO-30°C), as might be experienced during emersion on a European tidal flat, will not significantly affect the relationship between pB and ETR. However, care should be taken when using a single conversion factor between pB and ETR at the extremes of the temperature range. We have also shown that algal absorption measurements are important for correct calcUlation of ETR. The facts that different species seem to have different conversion factors and that changing environmental conditions will affect the absorption capacity and growth rate of the microphytobenthos (MPH) community suggest that it is wise to perform further calibrations of the relationship in the field before use in primary production modelling. Variable fluorescence measurements are quick and non-invasive and, with knowledge of the absorption properties of the MPH community, allow the quantification of photosynthetic parameters across large areas. Hence they are potentially useful for improving our estimates of ecosystem scale primary production.Key words: microphytobenthos. PAM fluorescence. photosynthesis, PSII electron transport. temperature Introduction Benthic microalgae or microphytobenthos (MPB) form highly productive natural ecosystems in intertidal areas (Cahoon, 1999). They are an important food source for both benthic and pelagic communities (MacIntyre et al., 1996; and have also been linked to sediment stabilization (Underwood et al., 1995).Three main factors appear to regulate the rate of photosynthesis in microphytobenthos: I~ght availability (Barranguet et al., 1998; Perkms et af., 2001), mud surface temperature (Blanchard et al., 1996; and CO2 availability (Underwood & Kromkamp, 1999 and references therein). Nutrients are not generally thought to be limiting in intertidal habitats (Underwood & . Both light and temperatureCorrespondence 10· E. Morris. e-mail: E.morris(a·nioo.knaw.nl change on seasonal, daily and hourly time scales. The two factors also co-vary, making their individual effects hard to separate in field situations . In order...

The widespread decline of seagrass beds within the Mediterranean often results in the replacement of seagrasses by opportunistic green algae of the Caulerpa family. Because Caulerpa beds have a different height, stiffness and density compared to seagrasses, these changes in habitat type modify the interaction of the seafloor with hydrodynamics, influencing key processes such as sediment resuspension and particle trapping. Here, we compare the effects on hydrodynamics and particle trapping of Caulerpa taxifolia, C. racemosa, and C. prolifera with the Mediterranean seagrasses Cymodocea nodosa and Posidonia oceanica. All macrophyte canopies reduced near-bed volumetric flow rates compared to bare sediment, vertical profiles of turbulent kinetic energy revealed peak values around the top of the canopies, and maximum values of Reynolds stress increased by a factor of between 1.4 (C. nodosa) and 324

No abstract

Dissolved organic nitrogen (DON) acts as a large reservoir of fixed nitrogen. Whereas DON utilization is common in the microbial community, little is known about utilization by macrophytes. We investigated the ability of the coexisting temperate marine macrophytes Zostera noltii, Cymodocea nodosa, and Caulerpa prolifera to take up nitrogen and carbon from small organic substrates of different molecular complexities (urea, glycine, L-leucine, and L-phenylalanine) and from dissolved organic matter (DOM) derived from algal and bacterial cultures (substrates with a complex composition). In addition to inorganic nitrogen, nitrogen from small organic substrates could be taken up in significant amounts by all macrophytes. Substrate uptake by the aboveground tissue differed from that of the belowground tissue. No relationships between carbon and nitrogen uptake of small organics were found. The preference for individual organic substrates was related to their structural complexity and C:N ratio. Uptake of algae-derived organic nitrogen was of similar magnitude as inorganic nitrogen, and was preferred over bacteria-derived nitrogen. These results add to the growing evidence that direct or quick indirect DON utilization may be more widespread among aquatic macrophytes than traditionally thought.KEY WORDS: Uptake · Dissolved organic nitrogen · Stable isotopes · Zostera noltii · Cymodocea nodosa · Caulerpa prolifera · Seagrasses · Macroalgae Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 427: [71][72][73][74][75][76][77][78][79][80][81] 2011 2008). But it is not yet known to what extent macrophytes share these capabilities. Although small organic compounds were until recently not considered a significant direct nutrient source for seagrasses (Romero et al. 2006), uptake of organic matter-derived nitrogen has been demonstrated for a number of macro algae and seagrass species (Bird et al. 1998, Tyler et al. 2003. Uptake of detritus-derived compounds by seagrasses was demonstrated by Evrard et al. (2005) and Barrón et al. (2006). However, these studies did not differentiate between direct DON uptake and uptake after remineralization by the bacterial community. On the other hand, macrophyte DON uptake in axenic cultures has been observed (Bird et al. 1998, Tarutani et al. 2004, suggesting that direct utilization of DON may be more common than is generally realized.Studying the uptake of natural DON (a complex mixture of organic nitrogen compounds) is inherently complicated. Not all compounds are treated the same way by primary producers. Tyler et al. (2005) demonstrated clear differences in the processing of 2 amino acids (alanine versus glycine) by a macroalga. Harrison et al. (2007) found relationships between amino acid complexity and uptake preference by terrestrial plants. Not all primary producers are equally capable of taking up organic nitrogen. Uptake capabilities for different compounds may even vary seasonally in certain microphytobenthic taxa (Nilsson & ...

No abstract