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Organic matter distribution and dynamics in marine ecosystems. Field, experimental and remote sensing approaches

Year: 2011
Type: dissertation
Abstract: The ocean is one of the largest reservoirs of reactive carbon on Earth surface. The pool of dissolved inorganic carbon is the greatest on the earth surface, with 38000 x106 Gt C, an order of magnitude greater than all other reactive pools (Hedges, 2002) and the global dissolved organic matter (DOM) pool in the ocean is estimated to contain 685 Gt of carbon, representing the second largest reactive carbon pools, a value comparable to the mass of inorganic carbon in the atmosphere. The carbon fluxes between the atmosphere and the ocean are about 70 x106 Gt C y-1; the same order as the exchange between the atmosphere and the terrestrial biota. Hence, small perturbations in the production or sink in the oceanic DOM pool would strongly affect the balance between oceanic and atmospheric CO2, and the processes that control DOM distribution and dynamics are significant with regard to carbon export and carbon storage in the deep ocean (Carlson, 2002). In this thesis, I have focused on two major pools of dissolved and particulate organic matter: chromophoric dissolved organic matter (CDOM) and transparent exopolymer particles (TEP). CDOM is a mixture of compounds that represent the optically active component of dissolved organic matter (DOM) at both visible and UV wavelengths (Nelson and Siegel, 2002; Coble, 2007). This DOM fraction can play a significant role in the cycling of carbon and other elements mediating photochemical reactions (Kieber et al., 1989; Mopper et al., 1991; Kieber et al., 1999) and it also plays a major role modulating the light transmission in the ocean, with consequences for protecting light-sensitive organisms from damage (Arrigo and Brown, 1996; Anderson et al., 2001; Williamson et al., 2001) or regulating photosyntetically active radiation availability (Kalle, 1966; Bricaud et al., 1981). In addition, in the past few years, the interest in CDOM studies has been promoted as it represents a significant component of ocean optical signals for satellite measurements of ocean color and can interfere on primary productivity satellite data (Siegel et al., 2002; Siegel et al., 2005) TEP are large, sticky particles, formed by acidic polysaccharides, and stainable with alcian blue (Alldredge et al., 1993). They are predominantly formed by spontaneous polymerization of dissolved precursors, mostly mono- and polysaccharides (DMCHO and DPCHO) released by microorganisms (Passow and Alldredge, 1994; Passow, 2000). TEP are a significant component of the sedimentary flux in marine ecosystems as they represent a interstitial matrix in coagulation processes to form larger, rapidly-sinking aggregates, known as marine snow, a major pathway for the vertical export of carbon to the deep ocean (Alldredge et al., 1993; Passow et al., 2001; Passow, 2002). In my thesis I have studied the dynamics of these two pools of organic matter using different approaches: direct field observations, experiments to determine sources and sinks, and long-term dynamics using remote sensing to assess the significance of the spatial and temporal scale. I have centered my research in two contrasting marine regions: The Antarctic Peninsula (Southern Ocean) and the Mediterranean Sea, with also a few data on the North Sea. The Southern Ocean ecosystem is characterized by an extreme seasonal variability in the incident solar radiation, low temperatures, and the influence of the annual advance and retreat of sea ice (Smetacek and Nicol, 2005; Ducklow et al., 2007). Despite it represents about a 10% of the total ocean surface, this ocean has a global impact on water mass circulation and carbon cycling through nutrient-rich deep water formation and subsequent meridional transfer as Antarctic Bottom water (Webb and Suginohara, 1997) and also as a significant sink of anthropogenic CO2 (Sarmiento et al., 1998; Takahashi et al., 2002). The Southern Ocean is considered as one of the largest HNLC (high nutrient-low chlorophyll) areas, that is, comparatively low algal biomass is found despite high nitrate and phosphate concentrations (Minas et al., 1986), most probably due to limitation by light (Smith et al 1992) or micronutrients such as iron (Debaar et al., 1995; Pakulski et al., 1996). The Mediterranean Sea is a semi-enclosed sea surrounded by Europe, Africa and Asia that represents the 0.7% of the ocean surface and 0.3% of the ocean volume. Despite its small volume, it shows thermohaline circulation and dense water formation processes. In addition, this area is unique in the sense that evaporation is greater than precipitation plus runoff (Chen et al., 2003) and has been one of the firsts where warming was evidenced and quantified (Bethoux et al., 1998) The main input of water to the Mediterranean Sea comes from the surface water from the Atlantic Ocean, depleted in nutrients, which flows through the strait of Gibraltar. This leads to the system to an oligothrophic status, so the Mediterranean sea can be considered as a Low Nutrient-Low chlorophyll area. Finally, I have also studied briefly some points of the North Sea, particularly in the German Bight where shallowness and river influence renders the area to high productivity.
Language: English
Subfield: Oceanography
Open Access status: green
OpenAlex ID: W52910222
Cites: 25
Cited by: 2
Related to: 10