Elsevier

Progress in Oceanography

Volume 81, Issues 1–4, April–June 2009, Pages 149-164
Progress in Oceanography

Ecosystem exploitation and trophodynamic indicators: A comparison between the Northern Adriatic Sea and Southern New England

https://doi.org/10.1016/j.pocean.2009.04.008Get rights and content

Abstract

In an ecosystem-based resource management context, it is crucial to assess the relationships between community structure and ecosystem function and how those relationships change with resource extraction. To elucidate how changes in resource use can affect community structure and ecosystem function, we executed a comparative analysis of two different ecosystems subjected to notable fishing pressure. We contrasted the Northern Adriatic Sea (NAS) and Southern New England (SNE) ecosystems by examining outputs from comparable steady-state models. Both ecosystems have relatively high fishing pressure and a high biomass of benthic invertebrates. The basic structure of the food webs shows differences both in the number and definition of the functional groups, as described in the models. Fisheries, on the contrary, show similarities both in terms of catches and discards. Almost all statistics summarizing the structure and flows showed values three times higher in the SNE than in the NAS ecosystem, but despite this difference the two ecosystems exhibited similar, overall properties. Biomass ratios and the Mixed Trophic Impact (MTI) analysis showed that both ecosystems are dominated by the benthic compartment. Removing the biomass effect, however, shows a clear top-down effect, with a high rank achieved by fishing activities. In general terms, the low mean trophic level of catches and the high primary production required (PPR) values result in a high overexploitation level of the ecosystem, as highlighted by the L index. We conclude by exploring how comparative studies will continue to be valuable as ecosystem-based management is further implemented.

Introduction

The ecosystem approach to natural resource management has been espoused (WSSD, 2002, FAO, 2003) and, at present, is recommended for adoption as a major policy initiative in Europe, the US, and elsewhere including the EU Water Framework Directive, Common Fisheries Policy and the European Marine Strategy (CBD, 2004, USOC, 2004, FAO, 2003). The aim of the new approach is to ensure that the planning, development and management of the environment will meet social and economic needs without jeopardizing options for future generations to benefit from the full range of goods and services provided by marine ecosystems; i.e. to ensure sustainable development (FAO, 2003, Pickitch et al., 2004). In this context, ecosystem-based management (EBM) aims to conserve the structure and function of ecosystems in order to maintain ecosystem services (CBD, 2004).

One of the main challenges for EBM is the implementation of this more holistic approach, in which the ecosystem represents the management unit (Raffaelli, 2006). Ecosystem considerations in a marine scientific and management context have been extant for more than a century (e.g. Baird, 1873), but how to make them operational has remained a key challenge. Research needs to be extended to encompass not only the structures of the ecosystem (classically studied components such as populations, species, communities, habitats) but also the processes related to the functioning of the ecological systems (production, consumption, respiration, energy flow and cycling). Ultimately, we need to seek general relationships among patterns and processes at multiple spatial scales (e.g. Zajac, 1999, Hyrenbach et al., 2000).

The relationship between marine ecosystem structure and function has become increasingly prominent in the last decade (Duffy, 2006, Raffaelli, 2006). Yet moving from knowledge of ecosystem structure and function to understanding the relationships between them has been more challenging. Shifting from the assessment of the human-use effects on ecosystem structure towards the assessment of the modifications that these structural changes induce in the processes of perturbed ecosystems requires caution due to the uncertainty, long feedback times and highly nonlinear ecosystem responses to external perturbations (Holling et al., 1995). For example, understanding how changes in biodiversity affect ecosystem function requires integrating diversity within trophic levels (horizontal diversity) and across trophic levels (vertical diversity; including food chain length and omnivory). As another example, the relative importance of top-down or bottom-up trophic controls in continental shelf ecosystems has important implications for how ecosystems respond to perturbations (e.g. Frank et al., 2007). The need to improve our knowledge about the relationships between structure and function is critical for adopting effective EBM strategies and policies.

One way to facilitate a better understanding of the relationship between ecosystem structure and function (and responses to perturbations thereupon) is to engage in comparative ecosystem studies (Hunt and Megrey, 2005, Moloney et al., 2005, Coll et al., 2007). Comparison between similar or comparable ecosystems (i.e. systems with similar latitudinal location and characterized by similar environmental features/constraints) is a useful analytical approach which can allow us to better understand the mechanisms which drive the functioning of ecological systems. In comparable ecosystems, some features will be shared but others will be unique, and analysis of these similar and contrasting patterns and processes can reveal important drivers in each ecosystem. These comparative analyses provide an opportunity for taking a broader ecosystem perspective and permit the ability to draw generalizations important to successful implementation of EBM.

To better elucidate key marine ecosystem properties and to facilitate implementation of EBM, we examined two comparable marine ecosystems; the Northern Adriatic Sea (NAS) and Southern New England (SNE). The reason we chose the NAS and SNE ecosystems are that they both represent relatively shallow, continental shelf ecosystems with a high benthic biomass. Additionally, both have had a notable history of extracting living marine resources, share some physio-chemical features, and have had enough scientific studies to generate adequate databases required for this study. Our objectives in this work were to present summaries of systemic metrics from both ecosystems and to compare those metrics between the two ecosystems.

Section snippets

Ecosystem and model descriptions

The upper portion of the Adriatic Sea (NAS) (Fig. 1) constitutes the widest continental shelf in the Mediterranean Sea and, in the context of the generally oligotrophic conditions of the basin, represents a unique habitat (Pinardi et al., 2006). The NAS is a shallow (<35 m) semi-enclosed basin (14,178 km2), characterized by strong riverine outflows, mainly from the Po River, with high loads of organic matter and nutrients (Revelante and Gilmartin, 1983, Turley, 1999). The NAS is influenced by

Results

Both ecosystems were dominated by invertebrates in terms of biomass and production (Table 4). Ranking the major compartments in relation to their contribution to the total system biomass showed that for both ecosystems the highest rank is attained by the benthos (64% and 48% for NAS and SNE, respectively). Differences were seen in the secondary ranking, nekton for NAS and plankton for SNE (Table 4). These observations were reflected in the Benthos:Plankton and Nekton:Plankton ratios, which were

Contrasts between NAS and SNE

In general terms, results from the structure and trophic flow analysis of the two ecosystems studied here demonstrated congruence with previous studies carried out in both ecosystems. The South New England (SNE) ecosystem is confirmed as one of the most productive marine areas on the planet (Bax, 1991, Cohen et al., 1982, Grosslein et al., 1980, Sissenwine et al., 1984); whereas, the Northern Adriatic Sea (NAS) system is confirmed as one of the most productive areas within the Mediterranean

Acknowledgments

We thank the CNR (Consiglio Nazionale delle Ricerche – Short term mobility program) for providing a travel fellowship to F.P. which initiated this collaboration. We thank S. Gaichas and anonymous reviewers of prior versions of the article which improved its quality and content.

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