We undertook a quantitative literature review to assess implicit scaling choices made in the design of enclosed experimental aquatic ecosystems (mesocosms). A database was constructed with information on temporal scale, spatial scale, and other design characteristics for 360 experiments reported in the literature. We found that key scaling variables such as complete physical dimensions were often not reported. Overall mesocosm experiments had a median volume of 1.7 m 3 and median duration of 49 d. Volume and duration varied by habitat type, experimental treatment, number of trophic levels included, and the response variable under investigation. A number of experimental design characteristics varied with mesocosm size. For instance, characteristics that decreased with increasing mesocosm size included the number of replicates and treatments, and the degree of experimental control over species composition and the physical environment. We also found a bias towards scaling cylindrical containers for a constant ratio of radius to depth as size was increased. This design choice inevitably alters surface-area-to-volume relationships as well as associated ecological variables such as gas and light energy flux, benthic-pelagic coupling, and the relative dominance of periphyton growth on mesocosm walls. Our results indicate the need for both 'scale sensitive' experiments, that explicitly consider scale in design and interpretation of results, and multi-scale' experiments that manipulate temporal and spatial attributes in order to test specific hypotheses regarding the ecological impact of modification in scale. Both types of experiments are prerequisites for improved mesocosm design and for the systematic extrapolation of information from experimental ecosystems to nature.
Abstract Economic valuation of ecological restoration most often encompasses only the most tangible ecosystem service benefits, thereby omitting many difficult‐to‐measure benefits, including those derived from enhanced reliability of ecosystem services. Because climate change is likely to impose novel ecosystem stressors, a typical approach to valuing benefits may fail to capture the contribution of ecosystem resilience to sustaining long‐term benefits. Unfortunately, we generally lack predictive probabilistic models that would enable measurement and valuation of resilience. Therefore, alternative measures are needed to complement monetary values and broaden understanding of restoration benefits. We use a case study of Chesapeake Bay restoration (total maximum daily load) to show that ecosystem service benefits that are typically monetized leave critical information gaps. To address these gaps, we review evidence for ecosystem services that can be quantified or described, including changes in harmful algal bloom risks. We further propose two integrative indicators of estuarine resilience—the extent of submerged aquatic vegetation and spatial distribution of fish. Submerged aquatic vegetation extent is indicative of qualities of ecosystems that promote positive feedbacks to water quality. Broadly distributed fish populations reduce risk by promoting diverse responses to spatially heterogeneous stresses. Our synthesis and new analyses for the Chesapeake Bay suggest that resilience metrics improve understanding of restoration benefits by demonstrating how nutrient and sediment load reductions will alleviate multiple sources of stress, thereby enhancing the system's capacity to absorb or adapt to extreme events or novel stresses.
Responses and acclimation of the submersed vascular plant Potamogeton perfoliatus L. to changes in total irradiance were investigated by growing replicate plant populations under neutral density screens to create three treatment levels (11, 32, and 100% of ambient). Changes in the relationship between photosynthesis and irradiance (P—I) were monitored during a 17—d treatment period and to 16—d "recovery" period, as were concentrations of photosynthetic pigments and several morphological features. Both initial slope of P—I relations and leaf chl a content increased significantly within 3 d after the beginning of shade treatment. These responses, which represent mechanisms of increasing photosynthetic efficiency at low irradiance, were also reversed within 3 d after treatment removal. Significant morphological responses to the recovery from shade were evident within 10 d, including: elongation of stems, thinning of lower leaves, and canopy formation at the water surface. Preliminary calculations indicate that both photosynthetic and morphological acclimations to shade conferred substantial improvements in P. perfoliatus production at experimentally reduced irradiance compared to pretreatment conditions. Significant decreases in plant stem density, biomass, and reproduction, as well as increases in mortality, were observed for plants at low (but not medium) growth irradiance. The inability of populations treated at low irradiance to exhibit any recovery (i.e., posttreatment increases) in these variables after 16 d of full ambient light suggests that 11% of ambient irradiance was below the minimum level needed for survival of this plant. Time scales for significant shade acclimation responses were comparable to the temporal scales of changes in light climate for aquatic ecosystems such as Chesapeake Bay. It is argued that, although stem elongation is a beneficial response to shade for P. perfoliatus in turbid lakes, it may be nonadaptive in turbulent tidal waters because of increased susceptibility to fragmentation.
