Dorte Krause-Jensen

Patterns of ammonium uptake within dense mats of the filamentous macroalga Chaetomorpha linum

Abstract The effect of macroalgal uptake on the flux of ammonium across the sediment-water interface was tested in laboratory experiments in which dense mats of Chaetomorpha linum were incubated at high and low surface irradiances and were exposed to a high simulated sediment nutrient flux. Depth profiles of NH + 4 concentrations within the 15-cm deep mats and the timing and magnitude of NH + 4 efflux through the mats to the overlying water reflected differences in macroalgal uptake between the two light treatments. Patterns of algal productivity and NH + 4 uptake with depth in the mats were determined from the accumulation of 13 C and 15 N in the algal tissue. Nitrogen-saturated macroalgae incubated at low irradiance exhibited a strong diel periodicity in NH + 4 uptake that was not present in the N-limited macroalgae incubated at high irradiance. Assimilation by the macroalgal mat at high irradiance was approximately 900 NH + 4 μmol m −2 h −1 , and was sufficient to prevent NH + 4 diffusion from the benthic nutrient source into the overlying water during both the light and dark periods. Uptake of NH + 4 in excess of the N growth demand in the lower half of the high-light mat resulted in a spatial separation of nutrient and light resources; NH + 4 did not diffuse into the upper layers and the most photosynthetically-active macroalgae remained N-deficient. Reduced irradiance decreased the total uptake of the mat by more than 50% (400 NH + 4 μmol m −2 h −1 ), and an efflux of NH + 4 into the overlying water occurred in the dark and early part of the light period. Ammonium diffused through the unproductive bottom layers of the low-light mat and was incorporated primarily in the photic zone in the upper 4 cm of the mat where photosynthesis provided the carbon required for N uptake and assimilation. These results support the hypothesis that actively-growing macroalgal mats efficiently sequester benthic nutrient inputs to the overlying water and reduce nutrient availability to a level that may limit pelagic production. Factors that reduce irradiance within the mat, such as self-shading or decreased insolation, limit macroalgal uptake of benthic flux and result in a release of nutrients into the overlying water.

Dynamics of seagrass stability and change

To the casual observer, seagrass meadows often appear to be uniform landscapes with limited structure. Belying this appearance, seagrass meadows contain considerable structure and dynamics (cf. den Hartog, 1971). Seagrass meadows, at any one time, consist of a nested structure of clones, possibly fragmented into different ramets, each supporting a variable number of shoots. Thus, although apparently rather static, seagrass meadows are highly dynamic landscapes maintained through the continuous recruitment of new clones to the meadow, and the growth and the turnover of the shoots they contain. Therefore, the intense dynamics of seagrass ecosystems results from the combination of processes operating at various scales, which—if balanced—maintain a rather stable ecosystem. Often, however, the various processes responsible for meadow dynamics are either unbal-

Connecting the dots: Responses of coastal ecosystems to changing nutrient concentrations

Empirical relationships between phytoplankton biomass and nutrient concentrations established across a wide range of different ecosystems constitute fundamental quantitative tools for predicting effects of nutrient management plans. Nutrient management plans based on such relationships, mostly established over trends of increasing rather than decreasing nutrient concentrations, assume full reversibility of coastal eutrophication. Monitoring data from 28 ecosystems located in four well-studied regions were analyzed to study the generality of chlorophyll a versus nutrient relationships and their applicability for ecosystem management. We demonstrate significant differences across regions as well as between specific coastal ecosystems within regions in the response of chlorophyll a to changing nitrogen concentrations. We also show that the chlorophyll a versus nitrogen relationships over time constitute convoluted trajectories rather than simple unique relationships. The ratio of chlorophyll a to total nitrogen almost doubled over the last 30–40 years across all regions. The uniformity of these trends, or shifting baselines, suggest they may result from large-scale changes, possibly associated with global climate change and increasing human stress on coastal ecosystems. Ecosystem management must, therefore, develop adaptation strategies to face shifting baselines and maintain ecosystem services at a sustainable level rather than striving to restore an ecosystem state of the past.

Oxygen and nutrient dynamics within mats of the filamentous macroalga Chaetomorpha linum

Concentration profiles of O2, NH4+, NO3−, and PO43− were measured at high spatial resolution in a 12-cm thick benthic mat of the filamentous macroalga Chaetomorpha linum. Oxygen and nutrient concentration profiles varied depending on algal activity and water turbulence. High surface irradiance stimulated O2 production in the surface layers and introduced O2 to deeper parts of the mat while the bottom layers of the mat and the underlying sediment were anoxic. Nutrient concentrations were highest in the bottom layers of the mat directly above the sediment nutrient source and decreased towards the surface layers due to algal assimilation and enhanced mixing with the overlying water column. Increased turbulence during windy periods resulted in more homogeneous oxygen and nutrient concentration profiles and shifted the oxic-anoxic interface downward. Denitrification within the mat, as measured by the isotope pairing technique on addition of 15NO3−, was found to take place directly below the oxic-anoxic interface. Denitrification activity was always due to coupled nitrification-denitrification, whereby nitrifiers in the mat utilize NH4+ diffusing from below and O2 diffusing from above. The denitrification rate in the mat ranged from 22 μmol m−2 h−1 to 28 μmol m−2 h−1, approximately equivalent to that measured in the surrounding nonvegetated sediment. Although sediment denitrification is suppressed when the sediment surface is covered by a dense macroalgal mat, the denitrification zone may migrate up into the mat. In eutrophic estuaries with a large area of macroalgal cover, the physical structure and growth stage of algal mats may thus play an important role in the regulation of nitrogen removal by denitrification.

Paradigms in the Recovery of Estuarine and Coastal Ecosystems

Following widespread deterioration of coastal ecosystems since the 1960s, current environmental policies demand ecosystem recovery and restoration. However, vague definitions of recovery and untested recovery paradigms complicate efficient stewardship of coastal ecosystems. We critically examine definitions of recovery and identify and test the implicit paradigms against well-documented cases studies based on a literature review. The study highlights a need for more careful specification of recovery targets and metrics for assessing recovery in individual ecosystems. Six recovery paradigms were identified and examination of them established that partial (as opposed to full) recovery prevails, that degradation and recovery typically follow different pathways as buffers act to maintain the degraded state, and that recovery trajectories depend on the nature of the pressure as well as the connectivity of ecosystems and can differ between ecosystem components and among ecosystems. A conceptual model illustrates the findings and also indicates how restoration efforts may accelerate the recovery process.

Diversity of European seagrass indicators: patterns within and across regions

Seagrasses are key components of coastal marine ecosystems and many monitoring programmes worldwide assess seagrass health and apply seagrasses as indicators of environmental status. This study aims at identifying the diversity and characteristics of seagrass indicators in use within and across European ecoregions in order to provide an overview of seagrass monitoring effort in Europe. We identified 49 seagrass indicators used in 42 monitoring programmes and including a total of 51 metrics. The seagrass metrics represented 6 broad categories covering different seagrass organizational levels and spatial scales. The large diversity is particularly striking considering that the pan-European Water Framework Directive sets common demands for the presence and abundance of seagrasses and related disturbance-sensitive species. The diversity of indicators reduces the possibility to provide pan-European overviews of the status of seagrass ecosystems. The diversity can be partially justified by differences in species, differences in habitat conditions and associated communities but also seems to be determined by tradition. Within each European region, we strongly encourage the evaluation of seagrass indicator–pressure responses and quantification of the uncertainty of classification associated to the indicator in order to identify the most effective seagrass indicators for assessing ecological quality of coastal and transitional water bodies.

Distribution, structure and function of Nordic eelgrass (Zostera marina) ecosystems: implications for coastal management and conservation

This paper focuses on the marine foundation eelgrass species, Zostera marina, along a gradient from the northern Baltic Sea to the north-east Atlantic. This vast region supports a minimum of 1480 km2 eelgrass (maximum >2100 km2), which corresponds to more than four times the previously quantified area of eelgrass in Western Europe. Eelgrass meadows in the low salinity Baltic Sea support the highest diversity (4–6 spp.) of angiosperms overall, but eelgrass productivity is low (<2 g dw m-2 d-1) and meadows are isolated and genetically impoverished. Higher salinity areas support monospecific meadows, with higher productivity (3–10 g dw m-2 d-1) and greater genetic connectivity. The salinity gradient further imposes functional differences in biodiversity and food webs, in particular a decline in number, but increase in biomass of mesograzers in the Baltic. Significant declines in eelgrass depth limits and areal cover are documented, particularly in regions experiencing high human pressure. The failure of eelgrass to re-establish itself in affected areas, despite nutrient reductions and improved water quality, signals complex recovery trajectories and calls for much greater conservation effort to protect existing meadows. The knowledge base for Nordic eelgrass meadows is broad and sufficient to establish monitoring objectives across nine national borders. Nevertheless, ensuring awareness of their vulnerability remains challenging. Given the areal extent of Nordic eelgrass systems and the ecosystem services they provide, it is crucial to further develop incentives for protecting them.

Testing the predictive power of seagrass depth limit models

The power of equations predicting seagrass depth limit (Zc) from light extinction (Kz) was tested on data on seagrass depth limits collected from the literature. The test data set comprised 424 reports of seagrass colonization depth and water transparency, including data for 10 seagrass species. This data set confirmed the strong negative relationship betweenZc andKz. The regression equation in Duarte (1991) overestimated the realized seagrass colonization depths at colonization depths < 5 m, while there was no prediction bias above this threshold. These results indicated that seagrass colonizing turbid waters (Kz 0.27 m-1) have higher apparent light requirements than those growing in clearer waters. The relationship between seagrass colonization depth and light attenuation shifts at a threshold of light attenuation of 0.27 m-1, requiring separate equations to predictZc for seagrass growing in more turbid waters and clearer waters, and to set targets for seagrass restoration and conservation efforts.

Distribution, structure and function of Nordic eelgrass (Zostera marina) ecosystems

This paper focuses on the marine foundation eelgrass species, Zostera marina, along a gradient from the northern Baltic Sea to the north-east Atlantic. This vast region supports a minimum of 1480 km2 eelgrass (maximum >2100 km2), which corresponds to more than four times the previously quantified area of eelgrass in Western Europe. Eelgrass meadows in the low salinity Baltic Sea support the highest diversity (4–6 spp.) of angiosperms overall, but eelgrass productivity is low (<2 g dw m-2 d-1) and meadows are isolated and genetically impoverished. Higher salinity areas support monospecific meadows, with higher productivity (3–10 g dw m-2 d-1) and greater genetic connectivity. The salinity gradient further imposes functional differences in biodiversity and food webs, in particular a decline in number, but increase in biomass of mesograzers in the Baltic. Significant declines in eelgrass depth limits and areal cover are documented, particularly in regions experiencing high human pressure. The failure of eelgrass to re-establish itself in affected areas, despite nutrient reductions and improved water quality, signals complex recovery trajectories and calls for much greater conservation effort to protect existing meadows. The knowledge base for Nordic eelgrass meadows is broad and sufficient to establish monitoring objectives across nine national borders. Nevertheless, ensuring awareness of their vulnerability remains challenging. Given the areal extent of Nordic eelgrass systems and the ecosystem services they provide, it is crucial to further develop incentives for protecting them.

Regulation of eelgrass (Zostera marina) cover along depth gradients in Danish coastal waters

A large data set, collected under the national Danish monitoring program, was used to evaluate the importance of photon flux density (PFD), relative wave exposure (REI), littoral slope, and salinity in regulating eelgrass cover at different depth intervals in Danish coastal waters. Average eelgrass cover exhibited a bell-shaped pattern with depth, reflecting that different factors regulate eelgrass cover at shallow- and deep-water sites. The multiple logistic regression analysis was used to identify regulating factors and determine their role in relation to eelgrass cover at different depth intervals. PFD, REI, and salinity were main factors affecting eelgrass cover while littoral slope had no significant effect. Eelgrass cover increased with increasing PFD at water depths of more than 2 m, while cover was in versely related to REI in shallow water. This pattern favored eelgrass cover at intermediate depths where levels of PFD and REI were moderate. Salinity had a minor, but significant, effect on eelgrass cover that is most likely related to the varying costs of osmoregulation with changing salinity. The analysis provided a useful conceptual framework for understanding the factors that regulate eelgrass abundance with depth. Although the regression model was statistically significant and included the factors generally considered most important in regulating eelgrass cover, its explanatory power was low, especially in shallow water. The largest discrepancies between predicted and observed values of cover appeared in cases where no eelgrass occurred despite sufficient light and moderate levels of exposure (almost 50% of all observations). These discrepancies suggest that population losses due to stochastic phenomena, such as extreme wind events, played an important regulating role that is not adequately described by average exposure levels. A more thorough knowledge of the importance of such loss processes and the time scales involved in recovery of seagrass populations after a severe disturbance are necessary if we are to understand the regulation of seagrass distribution in shallow coastal areas more fully.