Dissolved organic carbon (DOC) is a significant reservoir of carbon in the marine ecosystem. Consequently, understanding the processes governing DOC production and consumption is an important goal for the quantitative assessment of the global carbon cycle (Polimene et al., 2006). Marine ecosystem models are valuable tools to mechanistically investigate physiological and biogeochemical processes underpinning DOC dynamics.
According to the main aims of my PhD, I have conducted a preliminary study in order to assess the capacity of the European Regional Seas Ecosystem Model (ERSEM) to simulate the seasonal cycle of DOC observed at station L4 in the Western English Channel.
Area of study
L4 station (4 ° 13’ W 50 ° 15’ N) is located about 16 km southwest of Plymouth, in the Western English Channel (Figure 1), it has a maximum depth of 50 m and it is characterized by a strong seasonality with deep vertical mixing in winter and strong stratification during summer. Two distinct algal blooms are regularly observed, the first in spring, dominated by diatoms and the second in late summer, dominated by dinoflagellates (Widdicombe et al., 2010). Summer is characterized by low nutrient concentrations (Smyth et al., 2010).
Fig.1 Western English Channel, station L4 (Wyatt et al., 2010)
European Regional Seas Ecosystem Model (ERSEM)
ERSEM (Baretta et al., 1995; Blackford et al., 2004) is a biomass based marine biogeochemical model describing the cycling of carbon and nutrients (N, P, Si and Fe) within the lower trophic levels of the marine ecosystem. Model state variables include living organisms, dissolved nutrients, organic detritus, oxygen and CO2. Model living organisms are subdivided in three functional groups (Figure 2) describing the planktonic trophic chain: primary producers, consumers and decomposers. Primary producers and consumers are subdivided into 4 and 3 size-based functional types, respectively, while decomposers are modeled through only one functional type. More specifically, the phytoplankton community consists of picophytoplankton, nanoflagellates, dinoflagellates and diatoms. The zooplankton community includes: mesozooplankton, microzooplankton and heterotrophic nanoflagellates. Decomposers are modeled by one type of heterotrophic bacteria.
Functional types belonging to the same group share common process descriptions but different parameterizations.
A key feature of ERSEM is the decoupling between carbon and nutrient dynamics allowing the simulation of variable stoichiometry within the modeled organisms. Chlorophyll is also treated as an independent state variable following the formulation proposed by Geider et al., (1997). Consequently each plankton functional types is modeled throughout up to five state variables describing each cellular component (C, N, P, Si, Chl-a).
Fig.2 The pelagic ecosystem model flow diagram indicating the carbon and nutrient pathways between functional groups (Blackford et al., 2004)
Materials and methods
For this purpose, ERSEM was set-up as described in Polimene et al (2013). Furthermore, ERSEM was coupled with the General Ocean Turbulence Model (GOTM), forced with locally measured meteorological functions and run for six years (2002-2008).
Simulated concentrations of DOC and surface chlorophyll were monthly averaged and compared with available in situ data collected at L4 by the Western Channel Observatory (WCO).
Results and Conclusions
By comparing ERSEM and WCO climatology, both shown in Graph. 1, the results clearly indicate that ERSEM is able to qualitatively reproduce the observed seasonality of DOC and the observed temporal decoupling between chlorophyll (peaking in April) and DOC (peaking in July). The maximum DOC concentrations observed in summer are also well captured by the model while winter concentrations are underestimated. This result suggests a potentially important role of riverine DOC (which is not considered in the current model set-up) in determining winter DOC concentrations.
Graph.1 ERSEM climatology ranging from the year 2002 to the year 2008 for Total DOC and Chl WCO (Western Channel Observatory) climatology ranging from the year 2011 to the year 2012 for Total DOC and Chl
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