GLOBEC INTERNATIONAL NEWSLETTER APRIL 2007
An end to end approach in modelling Mediterranean pelagic ecosystems: can the Naples approach be a proof of feasibility? Javier Ruiz1, Maurizio Ribera d’Alcalà2, Alessandro Crise3 and Ioanna Siokou-Frangou4 1 Instituto de Ciencias Marinas de Andalucia, Cádiz, Spain (
[email protected]) 2 Stazione Zoologica ‘A. Dohrn’, Naples, Italy (
[email protected]) 3 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Sgonico, Italy (
[email protected]) 4 Hellenic Centre for Marine Research, Athens, Greece (
[email protected]) The Mediterranean and Black Seas embrace a diversity of habitats which resemble a wide spectrum of the world´s ocean habitats. Their prevailing oligotrophy (which may become extreme in the Levantine basin) is contrasted by the mesotrophic areas of convective deep mixing in the Gulf of Lion or the estuarinelike Northern Adriatic Sea, reaching the other extreme of anoxic conditions in the Black Sea.
resolution, even before resolving individual response, can be based on the constraints imposed by the size spectrum of the mesozooplankton and their prey. In addition, biological traits and tightly linked food preferences need to be taken into account. This led to five mesozooplankton functional groups being proposed as a balance between the simplification and complexity of the Mediterranean food webs.
A priori, this diversity does not seem to be the best arena in which to reach a consensus among experimentalists and modellers, but that is exactly what was achieved at the EUR-OCEANS M2EM (Mediterranean Marine Ecosystem Modelling) workshop (Fig. 1). More than thir ty modellers and experimentalists with experience in the Mediterranean and the Black Sea met in the Zoological Station of Naples for three Figure 1. Naples workshop participants. days. The latter gave their views on processes and interactions that they felt were crucial in ecosystem functioning whereas the former provided their vision on what is feasible, to date, to achieve from the modelling side. No tight schedule was programmed and elements were discussed in a very flexible environment, with time apportioned according to the debate established for each topic.
Copepods were considered as omnivorous filter feeders, similar to Euphausiids but with significant vertical migration, while Appendicularians were classified as filter feeders of small particles, and gelatinous carnivorous zooplankton (chaetognaths and macro-gelatinous plankton such as medusae or ctenophores) predating on fish larvae and zooplankton. Gelatinous filter feeders (salps and doliolids) filtering particles between 1 µm and 1 mm were also considered. Similar groups were considered for the Black Sea except that salps and doliolids can be ignored. Even though both experimentalists and modellers felt at ease with the choice, they were also aware of the difficulties in constructing equations for the groups defined. In particular, parameterising the size of the gelatinous carnivorous zooplankton is not straightforward because of the inherent presence of tentacles. Also, the gelatinous filter feeders have very patchy and sporadic dynamics. This may reflect strong nonlinearities which are difficult to simulate, but models can help to clarify their role in the ecosystem (e.g. Andersen and Nival, 1988).
From the start of the workshop, it was evident that it would be impossible to create a single, generic model capable of answering all the questions from end to end of the ecosystem and at all temporal and spatial scales. Instead, the group worked on the idea of embedding future advances into a flexible structure which resolved acceptably on specific processes and scales. One point was that the high diversity of the Mediterranean Sea makes the use of key species as ‘bricks’ for lower trophic levels inoperative. Therefore, the functional type approach, which is basically an ensemble average of schematic trophic behaviours, was taken as a good approximation of the response of the biota to habitat diversity up to the level of mesozooplankton. The current representation of mesozooplankton in biogeochemical models was considered to be over simplistic and not precise enough to simulate the transfer towards higher trophic levels. In the interplay between eating and being eaten, mesozooplankton relocate photosynthesized carbon into exploitable fish biomass. The size of food is a basic constraint for any predator and a key variable to resolve in this interplay. Increasing mesozooplankton
Although the development of mathematical homomorphisms for each functional type involves manifest difficulties, the use of new technological tools can make this task easier. Zooscan©, SIPPER© and other computer vision techniques applied to plankton are now complementing the picture of a devoted scientist attached to a binocular device. Their automatism is able to simulataneously provide size and some taxonomic sorting at processing speeds not envisaged in the past. Together with the exponential growth of computing power and automatism the information output from these new technologies will continue to increase in the future. Therefore, size resolved spatial and temporal patterns of mesozooplankton groups will become more readily available, thus enabling the validation of models which constrain them.
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ECOPATH ECOSYM
LAGRANGIAN
EULERIAN SMALL PELAGIC FISH KEY SPECIES
SIZE
MESOZOOPLANKTON small-large pelagic fish key species
SIZE FUNCTIONAL GROUP Dissolved gases oxygen carbon dioxide
Organic matter dissolved detritus
Microzooplankton
Nutrients
microzooplankton flagellates
Phytoplankton
Bacterioplankton
SEASONAL ANNUAL INTERANNUAL
GLOBEC INTERNATIONAL NEWSLETTER APRIL 2007
phosphate nitrate ammonium silicate reduced equivalents
diatoms flagellates picophytoplankton large phytoplankton
Figure 2. Schematic of the end to end met-modelling approach.
Fish mobility, behaviour and life cycles can impose temporal and spatial scales overriding those of advection and diffusion. In these cases, the Eulerian frame necessary to embed functional groups needs to be reconsidered for a model to be able to encompass the governing scales. To simulate ecosystem dynamics realistically at this level of complexity the life trajectory of individuals needs to be taken into account. This can be achieved using Lagrangian models based on these trajectories, the Individual Base Models (IBMs). In the workshop, small pelagic fish were considered as the border between Eulerian and Lagrangian approaches. An additional box can be included for them in an Eulerian frame or be modelled with IBMs. Rather than functional groups, key species were envisaged as the adequate focus at this level. Although IBMs can introduce biological elements not feasible in Eulerian schemes, they must be linked to the lower trophic dynamics simulated by Eulerian frames. The food allowing recruitment of exploitable species has its ultimate origin at these levels. This link must also be implemented to other frames like ECOPATH/ECOSIM in order to model top predators based on long term balances and network theory.
The conclusions from the Naples workshop provide a roadmap for future work and act as a guide for Mediterranean and Black Sea modellers to locate their inputs. The overall configuration is outlined in Figure 2 which depicts a meta-model already being adopted by new initiatives such as SESAME (Southern European Seas: Assessing and Modelling Ecosystem changes), a European Integrated Project of the Sixth Framework Programme. SESAME will develop equations for mesozooplankton and IBMs for targeted species as well as the connections between the different frames. SESAME aims to analyse past and future changes in the Mediterranean and Black Seas using models and will become an ideal testbed to check the implementation of this flexible frame in the heterogeneous oceanographic conditions present in the southern European Seas. Reference Andersen V. and P. Nival. 1988. A pelagic ecosystem model simulating production and sedimentation of biogenic particles - role of salps and copepods. Marine Ecology Progress Series 44(1): 37-50.
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