P.M.J. Herman

Ecosystem-based coastal defence in the face of global change

The risk of flood disasters is increasing for many coastal societies owing to global and regional changes in climate conditions, sea-level rise, land subsidence and sediment supply. At the same time, in many locations, conventional coastal engineering solutions such as sea walls are increasingly challenged by these changes and their maintenance may become unsustainable. We argue that flood protection by ecosystem creation and restoration can provide a more sustainable, cost-effective and ecologically sound alternative to conventional coastal engineering and that, in suitable locations, it should be implemented globally and on a large scale.

A model of early diagenetic processes from the shelf to abyssal depths

Abstract We present a numerical model of sedimentary early diagenetic processes that includes oxic and anoxic mineralization. The model belongs to the new wave of early diagenesis models that account for depth-dependent bioturbation and porosity profiles; it can be used both for calculating steady-state conditions and transient simulation. It was developed to reproduce the cycling of carbon, oxygen, and nitrogen along the ocean margin; it resolves the sediment-depth profiles of carbon, oxygen, nitrate, ammonium, and other reduced substances. Organic carbon is modeled as two degradable fractions with different first-order degradation rates and nitrogen:carbon ratios, to account for the decreasing reactivity and N/C ratio of the organic matter with depth into the sediment. The consumption of oxygen and nitrate as terminal electron acceptors is explicitly modeled, and mineralization is limited both by carbon (first order kinetics) and by oxidant availability (Michaelis-Menten type kinetics). Nitrification and oxic mineralization are decoupled, which allows the description of ammonium profiles. Mineralization processes using other oxidants (manganese oxides, iron oxides, sulphate) are lumped into one process, where degradation is only carbon limited; the terminal electron acceptors are not explicitly modeled, only the production of reduced substances is described. These substances are in part permanently removed (e.g., pyrite formation below the bioturbation zone) and partly diffuse towards the oxic layer where they react with oxygen. The values of several parameters were constrained using literature-derived relationships. The model was calibrated on a dataset obtained from the literature, which relates the magnitude of the different pathways to total organic carbon mineralization. The influence of carbon flux, bioturbation, sedimentation rate, bottomwater concentrations of oxygen, and nitrate and carbon degradability on the different mineralization pathways is examined. The relative contribution of the oxic mineralization in the model is significantly depressed under high organic flux, under low bottomwater oxygen conditions and when the bioturbation increases; higher carbon degradability has only a small positive effect, while sedimentation rate is relatively unimportant. Denitrification is mainly influenced by the nitrate concentration in the overlying bottomwater.

Ecology of estuarine macrobenthos

Macrobenthos is an important component of estuarine ecosystems. Based on a cross-system comparison, we show that estuarine macrobenthos may directly process a significant portion of the system-wide primary production, and that estuarine macrobenthic biomass may be predicted from primary production data. At large scales, food may be the prime limiting factor for benthic biomass. Depending on the characteristics of the system, grazing by benthic suspension feeders may be the most important factor determining system dynamics. The detailed spatial patterns and dynamics resulting from feeding interactions are discussed separately for suspension feeders and deposit feeders. The theory on local seston depletion and its consequences for spatial distribution of suspension feeders is compared critically with observed patterns of spatial distribution. It is concluded that additional non-linear interactions between biomass of the benthos and water currents must exist to explain the observed patterns. The relation between organic matter deposition fluxes and benthic community structure is discussed in the framework of the classical Pearson-Rosenberg paradigm. The importance of organic matter quality, in addition to quantity, is stressed. A simple model framework to investigate the relation between community structure and quantity of organic flux is proposed. Internal dynamics of benthic food webs are characterized by a high degree of omnivory (feeding on different trophic levels). This feature is contrasted with published data on food webs in other systems. It is hypothesized that the high quality of marine detritus (compared with terrestrial detritus) is the prime factor explaining the differences. Since theoretical studies suggest that omnivory destabilizes food webs, a number of stabilizing mechanisms in benthic food webs are discussed. Problems and mechanisms that could be explored fruitfully in theoretical studies and field comparisons are identified.

Denitrification in marine sediments: A model study

The rate and factors controlling denitrification in marine sediments have been investigated using a prognostic diagenetic model. The model is forced with observed carbon fluxes, bioturbation and sedimentation rates, and bottom water conditions. It can reproduce rates of aerobic mineralization, denitrification, and fluxes of oxygen, nitrate, and ammonium. The globally integrated rate of denitrification is estimated by this model to be about 230-285 Tg N yr(-1), with about 100 Tg N yr(-1) occurring in shelf sediments. This estimate is significantly higher than literature estimates (12-89 Tg N yr(- 1)), mainly because of a proposed upward revision of denitrification rates in slope and deep-sea sediments. Higher sedimentary denitrification estimates require a revision of the marine nitrogen budget and lowering of the oceanic residence time of nitrogen down to about 2 x 10(3) years and are consistent with reported low N/P remineralization ratios between 1000 and 3000 m. Rates of benthic denitrification are most sensitive to the flux of labile organic carbon arriving at the sediment-water interface and bottom water concentrations of nitrate and oxygen. Denitrification always increases when bottom water nitrate increases but may increase or decrease if oxygen in the bottom water increases. Nitrification is by far the most important source of nitrate for denitrification, except for organic-rich sediments underlying oxygen-poor and nitrate-rich water. [KEYWORDS: Organic-matter; biogeochemical cycles; early diagenesis; atmospheric co2; benthic fluxes; sea-floor; ocean; carbon;nitrogen; nutrient]

TRADE‐OFFS RELATED TO ECOSYSTEM ENGINEERING: A CASE STUDY ON STIFFNESS OF EMERGING MACROPHYTES

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.

Vegetation causes channel erosion in a tidal landscape

Vegetation is traditionally regarded to reduce the erosion of channels in both fl uvial and tidal landscapes. We present a coupled hydrodynamic, morphodynamic, and plant growth model that simulates plant colonization and channel formation on an initially bare, fl at substrate, and apply this model to a tidal landscape. The simulated landscape evolution is compared with aerial photos. Our results show that reduction of erosion by vegetation is only the local, on-site effect operating within static vegetation. Dynamic vegetation patches, which can expand or shrink, have a contrasting larger scale, off-site effect: they obstruct the fl ow, leading to flconcentration and channel erosion between laterally expanding vegetation patches. In contrast with traditional insights, our fi ndings imply that in tidal landscapes, which are colonized by denser vegetation, channels are formed with a higher channel drainage density. Hence this study demonstrates that feedbacks between vegetation, fl ow, and landform have an important control on landscape evolution.

Do alternate stable states occur in natural ecosystems? Evidence from a tidal flat

Studies from a wide variety of ecosystems indicate that primary producers may protect their environment against degrading processes such as erosion by water current or wind. Theoretical analyses showed that the dynamics of these systems are governed by positive feedback. We investigated the implications of a positive feedback between growth of benthic diatoms and erosion of silt in tidal flat systems. A simple mathematical model shows that alternate stable states may occur in systems with positive feedback between diatom growth and silt accumulation, particularly in sediments with intermediate bottom shear stress. High diatom cover, high silt content, and low levels of erosion characterize one state. The other state is dominated by erosion, and hence both diatom cover and silt content are low. In an experimental study, we tested the critical model assumption that the growth rate of diatoms increases with the silt content of the sediment. Net growth of diatoms was significantly higher on silt than on sandy sediment after nine days of incubation, supporting the premise that diatom-silt interactions are governed by positive feedback. Furthermore, we compared model predictions to data on the physical and biological prop- erties of sediments of a tidal flat. In accordance with our model, the silt content of sediments with intermediate to high bottom shear stress showed a clear and significant bimodal dis- tribution, which may reflect the existence of alternate stable states. At low bottom shear stress, silt content was better explained by a unimodal distribution, as was predicted by our model. Patterns in chlorophyll a content were less clear. Nevertheless, chlorophyll a content was best explained by a bimodal distribution at high bottom shear stress, and in two of the three periods at low bottom shear stress. Our study indicates that the positive feedback between enhanced production of diatoms and decreased erosion of sediment sig- nificantly affects the dynamics of intertidal flat systems.

Large-scale spatial patterns in estuaries: estuarine macrobenthic communities in the Schelde estuary, NW Europe

Few macrobenthic studies have dealt simultaneously with the two major gradients in estuarine benthic habitats: the salinity gradient along the estuary (longitudinal) and the gradients from high intertidal to deep subtidal sites (vertical gradient). In this broad-scale study, a large data set (3112 samples) of the Schelde estuary allowed a thorough analysis of these gradients, and to relate macrobenthic species distributions and community structure to salinity, depth, current velocities and sediment characteristics. Univariate analyses clearly revealed distinct gradients in diversity, abundance, and biomass along the vertical and longitudinal gradients. In general, highest diversity and biomass were observed in the intertidal, polyhaline zone and decreased with decreasing salinity. Abundance did not show clear trends and varied between spring and autumn. In all regions, very low values for all measures were observed in the subtidal depth strata. Abundance in all regions was dominated by both surface deposit feeders and sub-surface deposit feeders. In contrast, the biomass of the different feeding guilds showed clear gradients in the intertidal zone. Suspension feeders dominated in the polyhaline zone and showed a significant decrease with decreasing salinity. Surface deposit feeders and sub-surface deposit feeders showed significantly higher biomass values in the polyhaline zone as compared with the mesohaline zone. Omnivores showed an opposite trend. Multivariate analyses showed a strong relationship between the macrobenthic assemblages and the predominant environmental gradients in the Schelde estuary. The most important environmental factor was depth, which reflected also the hydrodynamic conditions (current velocities). A second gradient was related to salinity and confirms the observations from the univariate analyses. Additionally, sediment characteristics (mud content) explained a significant part of the macrobenthic community structure not yet explained by the two other main gradients. The different assemblages are further described in terms of indicator species and abiotic characteristics. The results showed that at a large, estuarine scale a considerable fraction of the variation in abundance and biomass of the benthic macrofauna correlated very well with environmental factors (depth, salinity, tidal current velocity, sediment composition).

Levy walks evolve through interaction between movement and environmental complexity

Animals’ movements may not only respond to the environment, but may also shape it, and thus affect fitness. Ecological theory predicts that animal movement is shaped by its efficiency of resource acquisition. Focusing solely on efficiency, however, ignores the fact that animal activity can affect resource availability and distribution. Here, we show that feedback between individual behavior and environmental complexity can explain movement strategies in mussels. Specifically, experiments show that mussels use a Lévy walk during the formation of spatially patterned beds, and models reveal that this Lévy movement accelerates pattern formation. The emergent patterning in mussel beds, in turn, improves individual fitness. These results suggest that Lévy walks evolved as a result of the selective advantage conferred by autonomously generated, emergent spatial patterns in mussel beds. Our results emphasize that an interaction between individual selection and habitat complexity shapes animal movement in natural systems.

Scale-Dependent Feedback and Regular Spatial Patterns in Young Mussel Beds

In the past decade, theoretical ecologists have emphasized that local interactions between predators and prey may invoke emergent spatial patterning at larger spatial scales. However, empirical evidence for the occurrence of emergent spatial patterning is scarce, which questions the relevance of the proposed mechanisms to ecological theory. We report on regular spatial patterns in young mussel beds on soft sediments in the Wadden Sea. We propose that scale‐dependent feedback, resulting from short‐range facilitation by mutual protection from waves and currents and long‐range competition for algae, induces spatial self‐organization, thereby providing a possible explanation for the observed patterning. The emergent self‐organization affects the functioning of mussel bed ecosystems by enhancing productivity and resilience against disturbance. Moreover, self‐organization allows mussels to persist at algal concentrations that would not permit survival of mussels in a homogeneous bed. Our results emphasize the importance of self‐organization in affecting the emergent properties of natural systems at larger spatial scales.