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Dissolved Organic Carbon Dynamics in the Ligurian Sea

1.1. Introduction

1.1.1. Why dissolved organic carbon?

Marine dissolved organic carbon (DOC), which is defined as a complex mixture of organic molecules that passes through a 0.2 µm filter, represents one of the largest (662 Pg C) [HAN 09] and the least understood reservoirs of organic carbon (C) on Earth. DOC contains as much carbon as the Earth's atmosphere; the net oxidation of 1% of its pool would therefore introduce an amount of CO2 to the atmosphere comparable to that released by the fossil fuel burning over one year. Most DOC is autochthonous, i.e. it is produced in situ by photosynthesis and chemosynthesis and released by phytoplankton, grazers (through egestion, excretion, sloppy feeding), dissolution of particles and viral lysis (CAR15). External sources (atmosphere, rivers, groundwater and sediments) may strongly affect DOC concentration and distribution, particularly in marginal semi-enclosed basins such as the Mediterranean Sea (Med Sea).

DOC also has great ecological significance as it represents the main source of energy for microbes (see Volume 1, Chapter 7 of this book series). Its consumption by heterotrophic prokaryotes fuels the microbial loop that, depending on the growth efficiency of both prokaryotic heterotrophs (BGE: bacterial growth efficiency) and their grazers, can represent a link or a sink of C for the food web. If most of the removed DOC is transformed into biomass by heterotrophic prokaryotes (high BGE), it is made available for grazers and can therefore be channeled again into the food web. If most of the removed DOC is mineralized to CO2 and inorganic nutrients (low BGE), the microbial loop represents a source of CO2 to the atmosphere and therefore a sink of C for the food web [AZA 83, FEN 08, SHE 88]. This scenario is further complicated by viruses that shift most of the energy towards the dissolved phase with modeled paradoxical effects on microbial growth, i.e. the lysis of a fraction of heterotrophic prokaryotes supports the growth of the community due to the release of labile molecules [FUH 09, FUH 99].

The oxidation of organic matter, both in its particulate and dissolved form, is responsible for oxygen consumption. Respiration is therefore tightly coupled to organic matter removal, and it is one of the major components of the carbon flux in the biosphere. Large uncertainties are associated with the estimate of respiration magnitude, as well as with our capacity to predict its response to global change.

Although marine DOC is produced and removed by many different processes, its concentration always falls within a very narrow range (34-80 µM C) [CAR 10, CAR 15, HAN 09, HAN 13]. The biogeochemical feedbacks that buffer DOC concentrations are unknown, and their understanding is one of the most intriguing and pressing issues in marine science. DOC can be considered as a dynamically stable reservoir of energy for the marine ecosystem, where all the energy that escapes from the food web can accumulate and be used when and where the ecosystem needs.

1.1.1.1. Biological lability of dissolved organic carbon

One of the most interesting aspects of DOC dynamics is that it includes molecules with a wide range of biological lability. Different fractions of DOC have therefore been described based on their turnover time. Labile DOC (LDOC) is defined as the fraction that is immediately used by prokaryotic heterotrophs and does not accumulate [HAN 13]. It therefore has a very low steady-state concentration, even though its production and removal rates are the highest ones. The fraction of DOC, which escapes rapid mineralization and accumulates, is considered to be recalcitrant. At least four fractions have been distinguished in this pool, depending on their lifetimes: Semi-Labile DOC (SLDOC, lifetime approximately 1.5 years), Semi-Refractory DOC (SRDOC, lifetime approximately 20 years), Refractory DOC (RDOC, lifetime 16,000 years) and Ultra-Refractory DOC (URDOC, lifetime approximately 40,000 years) [HAN 13]. Deep oceanic DOC is supposed to be constituted by RDOC+URDOC; however, Follet et al. [FOL 14] support the idea that a large portion (up to 30%) of DOC in deep waters has a modern radiocarbon age and a fast turnover time, which may be supported by particle dissolution. The new idea introduced in their paper is that, when particles are mineralized, not only LDOC is released but also SLDOC and SRDOC, which can resist for months to years, explaining the modern radiocarbon of a large portion of DOC in the deep waters.

1.1.1.2. Chromophoric dissolved organic matter

Chromophoric dissolved organic matter (CDOM) is the fraction of DOM that absorbs light at UV and visible wavelengths, and it represents the main factor determining the underwater light availability in open ocean and coastal waters. A fraction of CDOM also re-emits light as fluorescence and is defined as fluorescent DOM (FDOM). The absorption of the UV and blue portions of the solar radiation leads to the photodegradation of CDOM into small compounds (low molecular weight, low internal energy), with a change in its biological lability [NEL 13]. The study of the optical properties (absorption and fluorescence) of CDOM can give qualitative information on the CDOM pool, such as (1) the occurrence of different chromophores, (2) the changes in chromophores due to photodegradation and/or microbial transformation, (3) the main sources of CDOM (marine vs. terrestrial), (4) average molecular weight and aromaticity degree of CDOM and (5) biological lability; there are papers reporting that humic-like substances can be a tracer of recalcitrant DOM [CAT 15, ZHA 17], even though this is a controversial topic.

1.1.2. Why dissolved organic carbon in the Ligurian Sea?

In the Med Sea, DOC shows concentrations and vertical profiles similar to the oceanic ones, with concentrations in the intermediate and deep waters equal to the lowest values found in the deep Atlantic and Pacific (36-42 µM) [SAN 15a]. The very low DOC values in the deep Med Sea were unexpected since the renewal time of deep waters in the basin is 10 times shorter (20-126 years [AND 88, ROE 91]) than the renewal time of oceanic waters. The first DOC isotope data show that DOC in Med Sea deep water has an average age of 4,500-5,100 years, 1,000 years older than in the deep Atlantic Ocean [SAN 15b]. These authors suggest that a substantial fraction (up to 45%) of what has traditionally been defined as RDOC, imported from the Atlantic Ocean to the Med Sea, can be removed on temporal scales of 126 years, thereby opening intriguing questions about DOC lability and cycling in the deep Med Sea.

Figure 1.1. Map of the Ligurian Sea, with the stations (red dots) where the samples for DOC were collected in 2003 (April 28th-May 12th and September 1st-16th). The stars indicate the position of the stations where DOC data are available in the literature (Table 1.1); the dotted lines refer to the boundaries of the basin proposed by Béthoux et al. [BET 98]. The arrows represent water flows expressed in m3 yr-1 (further details are reported in Volume 1, Chapter 3 of this book series). For a color version of this figure, see www.iste.co.uk/migon/mediterranean2.zip

The Ligurian Sea (Figure 1.1) is characterized by cyclonic circulation, a clear stratification cycle [AND 00, VID 00] and is a site of deep-water formation [BET 83]. It is one of the areas with the highest primary production rates of the Med Sea [DOR 09, MAY 16], leading to high POC and DOC production. The continental inputs are significant only through the atmosphere (for a detailed description of the characteristics of the basin, see Volume 1, Chapter 3 of this book series). Due to its hydrology, the basin is characterized by trophic conditions varying from mesotrophy in spring to oligotrophy in summer and fall [BET 98, MAR 02b]. It can therefore be considered a natural laboratory where the major processes (i.e. thermohaline circulation, deep-water formation, seasonal cycle of stratification, intense meso- and sub-mesoscale activity, external inputs, phytoplankton seasonal cycle and composition) affecting DOC concentrations and distribution can be studied.

Global change is expected to influence all the above processes (deep-water formation rates, stratification pattern, atmospheric deposition pattern and Saharan dust events) affecting both DOC cycling and the quality of DOC transferred from the atmosphere to the surface ocean, and from the surface to the dark ocean. The DYFAMED station (red star in Figure 1.1) is the only open ocean site, where deep-water time series is available for DOC in the Med Sea. Due to the cyclonic circulation of the basin, this station is located in an area where vertical processes dominate over lateral advection [AND 00, BET 98]. It therefore represents an ideal site where production and removal processes and vertical export of DOC can be studied.

1.2. Dissolved organic carbon vertical distribution in the Ligurian Sea

In order to summarize the present knowledge on DOC vertical distribution in the basin, all the data, collected monthly at the DYFAMED station between 1990 and 19941 [AVR 02] and [COP 93], and those collected in the Ligurian Sea in May [SAN 10] and September 2003 (Figure 1.1), were averaged by depth (Figure 1.2) and compared to all the data available for the Ligurian...