A box model of carrying capacity for suspended mussel aquaculture in Lagune de la Grande-Entrée,Iles-de-la-Madeleine,Québec

An object-oriented model of environment–mussel aquaculture interactions and mussel carrying-capacity within Lagune de la Grande-Entrée (GEL), Iles-de-la-Madeleine, Québec, was constructed to assist in development of sustainable mussel culture in this region. A multiple box ecosystem model for GEL tied to the output of a hydrodynamic model was constructed using Simile software, which has inherent ability to represent spatial elements and specify water exchange between modelled regions. Mussel growth and other field data were used for model validation. Plackett–Burman sensitivity analysis demonstrated that a variety of bioenergetic parameters of zooplankton and phytoplankton submodels were important in model outcomes. Model results demonstrated that mussel aquaculture can be further developed throughout the lagoon. At present culture densities, phytoplankton depletion is minimal, and there is little food limitation of mussel growth. Results indicated that increased stocking density of mussels in the existing farm will lead to decreased mass per individual mussel. Depending on the location of new farm emplacement within the lagoon, implementation of new aquaculture sites either reduced mussel growth in the existing farm due to depletion of phytoplankton, or exhibited minimum negative impact on the existing farm. With development throughout GEL, an excess of phytoplankton was observed during the year in all modelled regions, even at stocking densities as high as 20 mussels m⁻³. Although mussels cultured at this density do not substantially impact the ecosystem, their growth is controlled by the flux of phytoplankton food and abundance of zooplankton competitors. This model provides an effective tool to examine expansion of shellfish farming to new areas, balancing culture location and density.     

Sustainable management of shellfish resources in Bandon Bay, Gulf of Thailand

Bandon Bay (Surat Thani Province) is one of the most productive coastal areas in southern Thailand. The Tapi River and 18 channels are the main sources of freshwater, nutrients, organic matter and sediment to the bay and the loading of freshwater and nutrients provide essential support for the production of phytoplankton in the estuarine ecosystem. Bandon Bay is important as natural spawning, nursery and feeding grounds for shellfish such as oysters, blood cockles, green mussels, short-necked clams, mud crabs and shrimps, and the estuary also serves as an excellent area for mariculturing of shellfish. In fact, oysters and blood cockles cultured in Bandon Bay are now being exported worldwide. However, Bandon Bay is also a textbook example of overexploitation of coastal resources in the tropics including all the derived changes in the estuarial ecosystem with severe socio-economic consequences. Hence, there is an urgent need for setting up an integrated management plant for a sustainable use of shellfish resources in Bandon Bay. The present study attempts to integrate water quality simulation results, socio-economic data and information on existing shellfish resource use in the process of proposing a set of sustainable management strategies for shellfish resources in Bandon Bay. These strategies involve: (1) using water quality modeling to monitor ecological and environmental changes in shellfish culture beds and their natural habitats in the process of setting up a master plan for management of waste water discharge into Bandon Bay; (2) zoning of shellfish mariculture in the coastal area in order to solve conflicts between resource users; (3) setting up a clear system for taxation of mariculture where the revenue may be used for (4) setting up and managing mangrove strips as filters of pollution and sediment around Bandon Bay; and finally (5) it is suggested to form a committee with members representing all relevant stakeholders plus the local government in order to work on resolving the existing and potential future conflicts over resource usage in Bandon Bay. This methodology may be seen as an important contribution towards a Bandon Bay sustainable management approach, based on the principles of integrated coastal zone management because it is science-based and takes into consideration the needs and perceptions of people involved in coastal resource extraction.     

Modeling the role of macroalgae in a shallow sub-estuary of Narragansett Bay, RI (USA)

Proliferation of macroalgal mats is a frequent consequence of nutrient-driven eutrophication in shallow, photic coastal marine ecosystems. These macroalgae have the potential to significantly modify water quality, plankton productivity, nutrient cycling, and dissolved oxygen dynamics. We developed a model for Ulva lactuca and Gracilaria tikvahiae in Greenwich Bay, RI (USA), a shallow sub-estuary of Narragansett Bay, as part of a larger estuarine ecosystem model. The model predicts the biomass of both species in units of carbon, nitrogen, and phosphorus as a function of primary production, respiration, grazing, decay, and physical exchange, with particular attention to the effects of biomass layering on light attenuation and suppression of metabolic rates. The model successfully reproduced the magnitude and seasonal cycle of area-weighted and peak biomass in Greenwich Bay along with tissue C:N ratios, and highlighted the importance of grazing and inclusion of self-limitation primarily in the form of self-shading to overcome an order of magnitude difference in rates of production and respiration. Inclusion of luxury nutrient uptake demonstrated the importance of internal nutrient storage in fueling production when nutrients are limiting. Macroalgae were predicted to contribute a small fraction of total system primary production and their removal had little effect on predicted water quality. Despite a lack of data for calibration and a fair amount of sensitivity to individual parameter values, which highlights the need for further autecological studies to constrain formulations, the model successfully predicted macroalgal biomass dynamics and their role in ecosystem functioning. Our formulations should be exportable to other temperate systems where macroalgae occur in abundance.     

The effects of intensive aquaculture on nutrient residence time and transport in a coastal embayment

Aquaculture in many countries around the world has become the biggest source of seafood for human consumption. While it alleviates the pressure on wild capture fisheries, the long-term impacts of large-scale, intensive aquaculture on natural coastal systems need to be better understood. In particular, aquaculture may alter habitat and exceed the carrying capacity of coastal marine ecosystems. In this paper, we develop a high-resolution numerical model for Sanggou Bay, one of the largest kelp and shellfish aquaculture sites in Northern China, to investigate the effects of aquaculture on nutrient transport and residence time in the bay. Drag from aquaculture is parameterized for surface infrastructure, kelp canopies, and bivalve cages. A model for dissolved inorganic nitrogen (DIN) includes transport, vertical turbulent mixing, sediment and bivalve sources, and a sink due to kelp uptake. Test cases show that, due to drag from the dense aquaculture and thus a reduction of horizontal transport, kelp production is limited because DIN from the Yellow Sea is consumed before reaching the interior of the kelp farms. Aquaculture drag also causes an increase in the nutrient residence time from an average of 5 to 10 days in the middle of Sanggou Bay, and from 25 to 40 days in the shallow inner bay. Low exchange rates and a lack of DIN uptake by kelp make these regions more susceptible to phytoplankton blooms due to high nutrient retention. The risk is further increased when DIN concentrations rise due to river inflows.     

Invasive species impacts on ecosystem structure and function: A comparison of Oneida Lake, New York, USA, before and after zebra mussel invasion

Exotic species invasion is widely considered to affect ecosystem structure and function. Yet, few contemporary approaches can assess the effects of exotic species invasion at such an inclusive level. Our research presents one of the first attempts to examine the effects of an exotic species at the ecosystem level in a quantifiable manner. We used ecological network analysis (ENA) and a social network analysis (SNA) method called cohesion analysis to examine the effect of zebra mussel (Dreissena polymorpha) invasion on the Oneida Lake, New York, USA, food web. We used ENA to quantify ecosystem function through an analysis of food web carbon transfer that explicitly incorporated flow over all food web paths (direct and indirect). The cohesion analysis assessed ecosystem structure through an organization of food web members into subgroups of strongly interacting predators and prey. Our analysis detected effects of zebra mussel invasion throughout the entire Oneida Lake food web, including changes in trophic flow efficiency (i.e., carbon flow among trophic levels) and alterations of food web organization (i.e., paths of carbon flow) and ecosystem activity (i.e., total carbon flow). ENA indicated that zebra mussels altered food web function by shunting carbon from pelagic to benthic pathways, increasing dissipative flow loss, and decreasing ecosystem activity. SNA revealed the strength of zebra mussel perturbation as evidenced by a reorganization of food web subgroup structure, with a decrease in importance of pelagic pathways, a concomitant rise of benthic pathways, and a reorganization of interactions between top predator fish. Together, these analyses allowed for a holistic understanding of the effects of zebra mussel invasion on the Oneida Lake food web.     

Importance of heterotrophic planktonic communities in a mussel culture environment: the Grande Entrée lagoon,Magdalen Islands (Québec,Canada)

Mussel culture in coastal environments relies on the availability of food of sufficient quality and quantity. Both to determine this availability and to examine impacts that this aquaculture practice may have on the environment, it is important to have good knowledge of the type of plankton communities present in aquaculture sites. It is usually thought that phytoplankton make up the bulk of mussel diet in many of these sites. Here we show that the Grande-Entrée lagoon [Magdalen Islands, Gulf of St Lawrence (GSL), Canada], where commercial mussel culture has been on-going since 1980, differs from this pattern. Heterotrophic protists dominate for most of the summer-early fall season (apart from short diatom bursts), with a high average biomass of 160 mg C m⁻³. The dominance of small-sized phytoplankton cells (notably green algae), low nutrient concentrations (e.g. 0.3 μM NO₃⁻ on average) and high biomass of heterotrophic protists (mostly naked ciliates and tintinnids) all point to the importance of the microbial food web in this shallow marine environment. Sustained cultivation of suspended mussels in the lagoon suggests that these heterotrophic protists could be an important source of food for the mussels, supplementing the small amount of phytoplankton present.     

Metabolism and model of an estuarine bay ecosystem affected by a coastal power plant

The effects of a coastal power plant on an outer estuarine bay ecosystem on the west coast of Florida were evaluated with measurements and an ecological model. Field measurements of community metabolism and biomass were taken from the thermally affected bay and from similar control bays. Model simulations were used to help understand these observations in terms of ecosystem structure and functioning.In the outer discharge bay the direct impact of the thermal plume was diluted and spread overlarge areas. The ecosystem developed structure and functions with lower biomass than in the control bays but with slightly faster rates of organic turnover. The productive turnover time of producer biomass during the summer was about 5 days in the discharge bay and about 6 days in the control bays. Power plant influence on total community metabolism was small with less than 10% difference in annual averages between the discharge and control bays (5.22 and 5.58 g O₂/m²/day). The selection for faster metabolic turnover rates in the discharge by was evidenced by a dominance of plankton metabolism over benthic metabolism. The annual average gross planktonic production was around 3 g O₂/m²/day in the discharge bay and around 2 g/m²/day in the control bays.In the model, temperature served as a stimulant to both productivity and respiration. When the isolated effects of increased temperature were simulated the model responded with lower producer biomass and faster rates of organic turnover, as was found in field measurements. These simulations also showed increased nutrient recycling and indicated patterns of temperature-induced migrations. Since power plant operation affected water exchange in the bays, several levels of total water exchange were simulated. These simulations indicated the importance of water exchange as a stabilizing factor, especially for sensitive compartments with rapid turnover rates (i.e. plankton and phosphorus stocks). Simulations of the effects of future power plant units on the bay ecosystem showed no large changes in total metabolism but indicated larger effects of plankton entrainment mortality and temperature-induced migrations of larger organisms.     

Lake of flies, or lake of fish? A trophic model of Lake Malawi

Ecosystem-focused models have, for the first time, become available for the combined demersal and pelagic components of a large tropical lake ecosystem, Lake Malawi. These provide the opportunity to explore continuing controversies over the production efficiencies and ecological functioning of large tropical lakes. In Lake Malawi these models can provide important insight to the effect of fishing on fish composition, and the potential competition that the lakefly Chaoborus edulis may have with fisheries production. A mass-balanced trophic model developed for the demersal fish community of the southern and western areas of Lake Malawi was integrated with an existing trophic model developed for the open-water pelagic. Input parameters for the demersal model were obtained from a survey of fish distributions, fish food consumption studies, and from additional published quantitative and qualitative information on the various biotic components of the community. The model was constructed using the Ecopath approach and software. The graphically presented demersal food web spanned four trophic levels and was based primarily on consumption of detritus, zooplankton and sedimented diatoms. Zooplankton was imported into the system at trophic levels three and four through fish predation on carnivorous and herbivorous copepods and Chaoborus larvae. It is proposed that the primary consumption of copepods was by fish migrating into the pelagic zone. Chaoborus larvae in the demersal were probably consumed near the lakebed as they conducted a daily migration from the pelagic to seek refuge in the sediments. This evidence for strong benthic-pelagic coupling provided the opportunity for linking the demersal model to the existing model for the pelagic community so producing the first model for the complete ecosystem. Energy fluxes through the resulting combined model demonstrated that the primary import of biomass to the demersal system was detritus of pelagic origin (72.1%) and pelagic zooplankton (10.6%). Only 15.8% of the biomass consumed within the demersal system was of demersal origin. Lakefly production is efficiently utilised by the lake fish community, and any attempt to improve fishery production through introduction of a non-native plantivorous fish species would have a negative impact on the stability and productivity of the lake ecosystem.     

Interaction Between Nutrient and Predator Limitation of Production in the Marine Euphotic Zone

Nutrient limitation of phytoplankton growth in nature is a complex phenomenon. the timing of nutrient limitation is a product of matching of algal growth with abiotic and/or biotic events regenerating nutrients, and mismatching with predator activity. the extent of production is governed by the concentration of atomic constituents which, in turn, is a function of the rapidity and quantity of nutrient regeneration by heterotrophs. Excess phytoplankton production over heterotroph demand is lost from the euphotic zone by sinking and from the ecosphere by sedimentation. Phytoplankton growth is therefore always limited by the size and activity of the regenerative food web, either directly through predation, or indirectly by inadequate nutrient regeneration. the open water column is a habitat deplete environment for metazoa, incapable of supporting simultaneous high predator and prey densities. Because of the incompatibility of the temporal and spatial scales of microbial and metazoan processes, and the presence of micro-habitats which can support a full recycling food web on microbial scales, the microbial loop is an important component of euphotic zone ecology. the total marine ecosystem runs at a nutrient sufficient level with nutrient deplete and replete phases dependent on matching of production with predation throughout the food web and subject to abiotic events. Man's release of N and P into coastal waters, if coupled with an increased incidence of mismatch resulting from climatic variation induced by the “greenhouse effect”, could have catastrophic effects on marine ecosystems.     

Quantifying the dynamics of marine invertebrate metacommunities: What processes can maintain high diversity with low densities in the Mediterranean Sea?

The Mediterranean Sea hosts 5.6% of the world benthic invertebrate species on 0.82% of the ocean surface. Mediterranean ecosystems are also characterized by low densities (and biomasses) compared to other oceanic ecosystems, a feature often attributed to their oligotrophic environment. Oligotrophic conditions can induce lower growth rates and higher mortality rates, and a stronger competition for food between individuals. A theoretical model was developed in order to study the diversity vs. density patterns in coastal benthic invertebrate species. This model describes their minimal population dynamics including basic processes (growth, mortality, reproduction and effects of competitive interactions between individuals) and incorporating fluxes of larvae (finally recruited as juveniles) between a mosaic of local habitats. Populations are therefore structured in a metacommunity. The connectivity between local communities is ensured by passive pelagic larval dispersal. In the Mediterranean Sea, because of the microtidal regime, the connectivity between coastal habitats is lower and more variable than in macrotidal basins. Mathematical properties of the model revealed that competitive interactions (intra- and interspecific competitions) have a stabilizing effect on interacting organisms when gains by recruitment are higher than losses by mortality. In addition, low mortality rates and low connectivity which decreases negative local interactions maintains high regional species diversity with low local densities. This property suggested that oligotrophy cannot be the only factor leading to the high diversity–low density pattern observed in the Mediterranean Sea.     

Multispecies perspective for small-scale fisheries management: A trophic analysis of La Paz Bay in the Gulf of California,Mexico

Given the complexity of small-scale fisheries and the difficulties for applying classical assessment methods, the status of these fisheries has been poorly documented. In this study, we used a trophic mass-balance model as an analytical alternative to evaluate the trophic impacts of small-scale fisheries as a whole on the marine ecosystem and their implications for ecosystem-based management, taking as a case study the La Paz bay and adjacent fishing grounds (BALAP) located in the Gulf of California, Mexico.Maturity indices, like ascendency and primary production to respiration ratio, indicate that the BALAP ecosystem is in a developing stage. This seems to be closely related to the reported two-season climatic regime that results in a nutrient supply characterized by an oscillating upwelling. The trophic model predicts a predominance of bottom-up control in the food web, which is congruent with the immaturity of the ecosystem. In this context, fisheries seem not to cause a significant impact to the ecosystem as a whole; however, target species show signals of being fully exploited by fisheries in the system. Red snapper and sharks showed the highest exploitation rates in the ecosystem. Based on these results, we discuss the current stock concept as a population-based management unit and the necessity for defining an ecosystem-based management unit.     

Meta-ecosystem engineering: nutrient fluxes reveal intraspecific and interspecific feedbacks in fragmented mussel beds

Ecologists still have to elucidate the complex feedback interactions operating among biodiversity and ecosystem processes in engineered systems. To address this, a field experiment was conducted to mimic natural mussel bed meta-ecosystems (Mytilus spp.) of the lower St. Lawrence Estuary (Quebec, Canada) and partition the effects of their biotic and abiotic properties and spatial structure on ecosystem processes and community dynamics of associated macro-invertebrates. We found positive intraspecific feedbacks between mussels and their recruits, and negative interspecific feedbacks between mussels and their associated ecosystem. These feedbacks were associated with mussel bed ecosystem processes (fluxes of ammonium and oxygen). In addition, we showed that proximity between mussel patches increased within-patch nutrient fluxes. Our study revealed the potential for meta-ecosystem engineering to drive feedback interactions between community and ecosystem functioning in marine fragmented systems. It also shows the relevance of meta-ecosystem theories as a conceptual framework to elucidate biotic and abiotic processes controlling ecosystem and community structure. Such framework could contribute to ecosystem-based management of spatially structured systems such as reserve networks and fragmented ecosystems.     

Shift in nutrient and plankton community in eutrophic lake following introduction of a freshwater bivalve

The impact of the freshwater bivalve Corbicula leana on plankton community dynamics was examined during a cyanobacterial bloom period. Nutrient and chlorophyll concentrations, primary productivity, and phytoplankton and zooplankton communities in the experimental enclosures were measured at 2-3 day intervals. The introduction of mussels reduced net primary productivity and phytoplankton and chlorophyll. Chlorophyll decreased immediately following addition of 100 mussels and then increased over time. After 600 mussels were added, chlorophyll decreased continuously from 87to 25 microg l(-1), approaching that in the mussel-free enclosure. Simultaneously, water transparency increased and concentrations of suspended solids and total phosphorus decreased. Mussel addition caused short-term increases in nutrient concentrations, especially following high-density treatment: phytoplankton density decreased, while cell density in the mussel-free enclosure increased. Zooplankton densities in the two enclosures were similar; however, carbon biomass in the mussel enclosure increased, associated with an increase in large zooplankton. The trophic relationship between phytoplankton and zooplankton was positive in the mussel-free enclosure and negative in the mussel-treatment enclosure, possibly reflecting effects of mussels on both consumer and resource control in the plankton community. Thus, filter feeding by Corbicula affects nutrient recycling and plankton community structure in a freshwater ecosystem through direct feeding and competition for food resources.     

湿地生态系统硅生物地球化学循环研究进展

硅在地壳中的丰度仅次于氧,是地球表面大多数土壤和岩石的一种基本成分,也是水生植物（特别是硅藻类）以及多种作物生长所必需的营养元素,还是控制陆地、海洋、沿海和内陆水生态系统机能的重要营养元素。目前关于全球硅的生物地球化学循环的研究多集中在陆地和海洋两大生态系统,而湿地生态系统中硅的循环过程、储存量尚不清楚。虽然在河口湿地开展一些关于硅的相关研究,但是硅在湿地的循环机制研究不够全面,尤其相比碳、氮、磷等元素,硅素研究甚少。而且,国内关于硅的相关研究更为匮乏。本文在总结国内外关于湿地生态系统硅素研究的基础上,综述硅在湿地生态系统的存在形态与分布特征,阐述硅在湿地生态系统中的基本循环过程,列举影响硅在湿地生态系统中循环的主要因素,如：湿地类型、水淹时间、季节变化、人类干扰等;提出在今后研究工作中应进一步探索硅在湿地生态系统中迁移、转化的机制,加深研究人类活动对湿地生态系统硅循环的影响,特别是应该加强研究河口潮汐湿地和沿海湿地生态系统硅的生物地球化学循环过程和储存量,有助于明确湿地生态系统对于硅的截留量;并弄清湿地中碳与硅含量之间的关系,从水文学角度分析湿地中排水、蒸发、洪期及滞留时间等因素对硅循环的影响,从而试图建立湿地硅循环模型,有助于预测湿地生态系统硅循环对沿海地区赤潮等富营养化现象和全球气候变化的影响。     

Differential tree and shrub production in response to fertilization and disturbance by coastal river otters in Alaska

We explored the interacting effects of marine-derived nutrient fertilization and physical disturbance introduced by coastal river otters (Lontra canadensis) on the production and nutrient status of pristine shrub and tree communities in Prince William Sound, Alaska, USA. We compared production of trees and shrubs between latrines and non-latrines, while accounting for otter site selection, by sampling areas on and off sites. Nitrogen stable isotope analysis (delta15N) indicated that dominant tree and shrub species assimilated the marine-derived N excreted by otters. In association with this uptake, tree production increased, but shrub density and nonwoody aboveground shrub production decreased. The reduced shrub production was caused by destruction of ramets, especially blueberry (Vaccinium spp.), through physical disturbance by river otters. False azalea (Menziesia ferruginea) ramets were less sensitive to otter disturbance. Although surviving individual blueberry ramets showed a tendency for increased production per plant, false azalea allocated excess N to storage in leaves rather than growth. We found that plant responses to animal activity vary among species and levels of biological organization (leaf, plant, ecosystem). Such differences should be accounted for when assessing the influence of river otters on the carbon budget of Alaskan coastal forests at the landscape scale.     

Benthic algal production across lake size gradients: interactions among morphometry, nutrients, and light

Attached algae play a minor role in conceptual and empirical models of lake ecosystem function but paradoxically form the energetic base of food webs that support a wide variety of fishes. To explore the apparent mismatch between perceived limits on contributions of periphyton to whole-lake primary production and its importance to consumers, we modeled the contribution of periphyton to whole-ecosystem primary production across lake size, shape, and nutrient gradients. The distribution of available benthic habitat for periphyton is influenced by the ratio of mean depth to maximum depth (DR = z/ z(max)). We modeled total phytoplankton production from water-column nutrient availability, z, and light. Periphyton production was a function of light-saturated photosynthesis (BPmax) and light availability at depth. The model demonstrated that depth ratio (DR) and light attenuation strongly determined the maximum possible contribution of benthic algae to lake production, and the benthic proportion of whole-lake primary production (BPf) declined with increasing nutrients. Shallow lakes (z < or =5 m) were insensitive to DR and were dominated by either benthic or pelagic primary productivity depending on trophic status. Moderately deep oligotrophic lakes had substantial contributions by benthic primary productivity at low depth ratios and when maximum benthic photosynthesis was moderate or high. Extremely large, deep lakes always had low fractional contributions of benthic primary production. An analysis of the world's largest lakes showed that the shapes of natural lakes shift increasingly toward lower depth ratios with increasing depth, maximizing the potential importance of littoral primary production in large-lake food webs. The repeatedly demonstrated importance of periphyton to lake food webs may reflect the combination of low depth ratios and high light penetration characteristic of large, oligotrophic lakes that in turn lead to substantial contributions of periphyton to autochthonous production.     

Decadal changes in a NW Mediterranean Sea food web in relation to fishing exploitation

We analysed changes in the ecological roles of species, trophic structure and ecosystem functioning using four standardized mass-balance models of the South Catalan Sea (North-western Mediterranean). Models represented the ecosystem during the late 1970s, mid 1990s, early 2000s, and a simulated no-fishing scenario. The underlying hypothesis was that ecosystem models should quantitatively capture the increasing exploitation in the ecosystem from the 1970s to 2000s, as well as differences between the exploited and non-exploited scenarios. Biomass showed a general decrease, while there was an increase in biomass at lower trophic levels (TL) from the 1970s to 2000s. The efficiency of energy transfer (TE) from lower to higher TLs significantly increased with time. The ecosystem during the 1990s showed higher biomass and flows than during the 1970s and 2000s due to an increase in small pelagic fish biomass (especially sardines). Exploited food webs also showed similarities in terms of general structure and functioning due to high intensity of fishing already in the 1970s. This intensity was highlighted with low trophic levels in the catch, high consumption of production by fisheries, medium to high primary production required to sustain the catches and high losses in secondary production due to fishing. Significant differences on ecosystem structure and functioning were highlighted between the exploited and no-fishing scenarios. Biomass of higher TLs increased under the no-fishing scenario and the mean trophic level of the community and the fish/invertebrate biomass ratios were substantially lower in exploited food webs. The efficiency of energy transfer (TE) from lower to higher TLs was lower under the no-fishing scenario, and it showed a continuous decrease with increasing TL. Marine mammals, large hake, anglerfish and large pelagic fish were identified as keystone species of the ecosystem when there was no fishing, while their ecological importance notably decreased under the exploited periods. On the contrary, the importance of small-sized organisms such as benthic invertebrates and small pelagic fish was higher in exploited food webs.     

Restoring piscivorous fish populations in the Laurentian Great Lakes causes seabird dietary change

Ecosystem change often affects the structure of aquatic communities thereby regulating how much and by what pathways energy and critical nutrients flow through food webs. The availability of energy and essential nutrients to top predators such as seabirds that rely on resources near the water's surface will be affected by changes in pelagic prey abundance. Here, we present results from analysis of a 25-year data set documenting dietary change in a predatory seabird from the Laurentian Great Lakes. We reveal significant declines in trophic position and alterations in energy and nutrient flow over time. Temporal changes in seabird diet tracked decreases in pelagic prey fish abundance. As pelagic prey abundance declined, birds consumed less aquatic prey and more terrestrial food. This pattern was consistent across all five large lake ecosystems. Declines in prey fish abundance may have primarily been the result of predation by stocked piscivorous fishes, but other lake-specific factors were likely also important. Natural resource management activities can have unintended consequences for nontarget ecosystem components. Reductions in pelagic prey abundance have reduced the capacity of the Great Lakes to support the energetic requirements of surface-feeding seabirds. In an environment characterized by increasingly limited pelagic fish resources, they are being offered a Hobsonian choice: switch to less nutritious terrestrial prey or go hungry.     

From single fine roots to a black alder forest ecosystem: How system behaviour emerges from single component activities

Based on empirical findings in a natural black alder ecosystem in Northern Germany we developed an individual based model that integrates components of a black alder ecosystem interacting on different levels of organisation. The factors determining seasonal fine root biomass development of forest ecosystems are not yet fully understood.We used an object oriented model approach to investigate this complex matter for black alder trees. Processes like growth, storage, respiration, transport, nutrient mineralisation and uptake as well as interactions among these factors are described on the level of functionally differentiated plant organs (fine roots, coarse roots, stem, branches, leaves) and soil units. The object structure of the model is determined by spatial relations between plant modules as well as between plant modules and their local environment modules.As results of model application we found that (i) on the organ level, spatio-temporal plasticity of (root) growth allocation is related to spatio-temporal variation of resource availability, (ii) on the plant level, balanced root:shoot growth appears in response to variation of available resources light and nutrients, (iii) on the population level, tree stand development (population structure, self-thinning) resulted from coexistence and competition between plant individuals.For the understanding of the root compartment it seems relevant that the model implementation of local scale fine root dynamics is consistent with a self-organised large scale spatial heterogeneity of fine root activity pattern. On the other hand, fine-root dynamics cannot be explained as a result of autonomous dynamics. A reference to above-ground processes is a necessary condition and the overall plant seems to act as an integrator providing boundary conditions for local activity pattern. At the same time fine-root characteristics are of some importance for properties on hierarchically higher levels, e.g. co-existence in a tree population or element cycling in the ecosystem.As a conclusion, modelling of the spatio-temporal dynamics of tree root systems appears as a paradigmatic example of scale and organisation level integrating processes.