Abstract. Biologically mediated modifications of the abiotic environment, also called ecosystem engineering, can significantly affect a broad range of ecosystems. Nevertheless, remarkably little work has focused on the costs and benefits that ecosystem engineers obtain from traits that underlie their ecosystem engineering capacity. We addressed this topic by comparing two autogenic engineers, which vary in the degree in which they affect their abiotic environment via their physical structure. That is, we compared two plant species from the intertidal coastal zone (Spartina anglica and Zostera noltii), whose shoots are exposed to similar currents and waves, but differ in the extent that they modify their environment via reduction of hydrodynamic energy. Our results indicate that there can be trade-offs related to the traits that underlies autogenic ecosystem engineering capacity. Dissipation of hydrodynamic forces from waves was roughly a factor of three higher in vegetation with stiff leaves compared to those with flexible leaves. Drag was highest and most sensitive to hydrodynamic forces in stiff vegetation that does not bend with the flow. Thus, shoot stiffness determines both the capacity to reduce hydrodynamic energy (i.e., proxy for ecosystem engineering capacity) and the drag that needs to be resisted (i.e., proxy for associated costs). Our study underlines the importance of insight in the trade-offs involved in ecosystem engineering as a first step toward understanding the adaptive nature of ecosystem engineering.

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.

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During the last decade, the Palmones River estualy has undergone severe eutrophication followed by a green tide episode; two species ofUlva, rotundata Blid. and Ulva curvata (Kutz.) De Toni, were the main macroalgae responsible for this bloom. From November 1993 to December 1994, we followed the biomass, the growth dynamics, and tissue elemental composition (C:N:P) of Ulva species, as well as some physicochemical variables in the estualy. Maximum biomass (up to 375 g dly wt.m-2 in some spots, corresponding to a thallus area index of nearly 17 m2 Ulva m-2 sediment) were observed in June and December. However, the biomass vaned among the sampling stations. Water nitrate, ammonia, and phosphate showed high concentrations throughout the year, with extremely high transient pulses, sustaining the high growth rates obseroed. Growth rates were estimated directly i n the field. The rates were generally higher i n Ulva discs maintained in net cages than those estimated by changes in biomass standing stock between two consecutive samplings. The dqerence between both estimates was used to quantijj the importance of the processes causing loss of biomass, which were attrib utable to grazing, exported biomass, and thallus decomposition under anaerobic conditions resulting from extreme selfshading. Maximum chlor@hyll content was found in winter, whereas the minimum was in spring. Atomic N:P ratios were generally higher in the algae than in the water, However, the absolute concentrations of tissue N and P were always higher than the cn'tical h e l s for maximum growth, which suggests that growth was not limited by inorganic N or P availability. The results suggested that the increase in nutrient loading in the river may have triggered the massive development of green algae and that light limitation and temperature stress in summer seem to be the main factors controlling the abundance of Ulva in the estualy. I n addition to light availability and thermal stress, the different loss processes may have a decisive role in the dynamics of Ulva biomass. K q index words: eutrophication; growth; primaly p r e duction; Ulva Local proliferations of masses of green algae are often found in estuaries and coastal waters of areas undergoing eutrophication (Geertz-Hansen et al.

The present study assessed the effect of seawater ammonium enrichment on survival and growth of the seagrass Zostera noltii Hornem. Ammonium enrichment had an inhibitory effect on shoot, rhizome and root elongation rates, as well as on primary production. The inhibitory effect was partially alleviated by phosphate addition. The frequency of ammonium pulses also affected growth negatively, with a maximum effect at both low and high pulse frequencies. Similar results were obtained when the experiment was run in the field during a winter trial. However, when the field experiment was repeated in spring at higher mean temperature and irradiance levels, opposite results were obtained, with ammonium enrichment causing a substantial increase in growth rates and productivity. These responses were related to the internal balance of non-structural carbohydrates (especially sucrose) with respect to C and N cell metabolism. Thus, there was a mobilisation of sucrose in both above-and belowground tissues to meet C increased demands arising from ammonium assimilation in winter, with sucrose concentrations reaching critical levels in relation to the total internal C pool. In contrast, sucrose accumulated in the tissues when ammonium pulses were applied in spring, indicating an enhanced C turnover that was able to meet the increased demands arising from ammonium assimilation into organic N compounds.

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Within the sheltered creeks of Cádiz bay, Ulva thalli form extended mat-like canopies. The effect of solar ultraviolet radiation on photosynthetic activity, the composition of photosynthetic and xanthophyll cycle pigments, and the amount of RubisCO, chaperonin 60 (CPN 60), and the induction of DNA damage in Ulva aff. rotundata Bliding from southern Spain was assessed in the field. Samples collected from the natural community were covered by screening filters, generating different radiation conditions. During daily cycles, individual thalli showed photoinhibitory effects of the natural solar radiation. This inhibition was even more pronounced in samples only exposed to photosynthetically active radiation (PAR). Strongly increased heat dissipation in these samples indicated the activity of regulatory mechanisms involved in dynamic photoinhibition. Adverse effects of UV-B radiation on photosynthesis were only observed in combination with high levels of PAR, indicating the synergistic effects of the two wavelength ranges. In samples exposed either to PAR+UV-A or to UV-B+UV-A without PAR, no inhibition of photosynthetic quantum yield was found in the course of the day. At the natural site, the top layer of the mat-like canopies is generally completely bleached. Artificially designed Ulva canopies exhibited fast bleaching of the top layer under the natural solar radiation conditions, while this was not observed in canopies either shielded from UV or from PAR. The bleached first layer of the canopies acts as a selective UV-B filter, and thus prevents subcanopy thalli from exposure to harmful radiation. This was confirmed by the differences in photosynthetic activity, pigment composition, and the concentration of RubisCO in thalli with different positions within the canopy. In addition, the induction of the stress protein CPN 60 under UV exposure and the low accumulation of DNA damage indicate the presence of physiological protection mechanisms against harmful UV-B. A mechanism of UV-B-induced inhibition of photosynthesis under field conditions is proposed.