Marine ecosystems' responses to climatic and anthropogenic forcings in the Mediterranean

Oceanography and marine biology: an annual review

Bottom trawling has many impacts on marine ecosystems, including seafood stock impoverishment, benthos mortality, and sediment resuspension. Historical records of this fishing practice date back to the mid-1300s. Trawling became a widespread practice in the late 19th century, and it is now progressively expanding to greater depths, with the concerns about its sustainability that emerged during the first half of the 20th century now increasing. We show here that compared with untrawled areas, chronically trawled sediments along the continental slope of the north-western Mediterranean Sea are characterized by significant decreases in organic matter content (up to 52%), slower organic carbon turnover (ca. 37%), and reduced meiofauna abundance (80%), biodiversity (50%), and nematode species richness (25%). We estimate that the organic carbon removed daily by trawling in the region under scrutiny represents as much as 60–100% of the input flux. We anticipate that such an impact is causing the degradation of deep-sea sedimentary habitats and an infaunal depauperation. With deep-sea trawling currently conducted along most continental margins, we conclude that trawling represents a major threat to the deep seafloor ecosystem at the global scale.

https://www.pnas.org/content/pnas/111/24/8861.full.pdf

Chronic and intensive bottom trawling impairs deep-sea biodiversity and ecosystem functioning

Tools used for assessing marine trophic status are generally based on water column characteristics, which, however, may provide unreliable classification of the benthic trophic status. Here, we provide evidence from the literature that quantity and bioavailability of sediment organic matter are reliable proxies to assess benthic marine trophic status. We compiled data on the protein, carbohydrate and lipid concentration of sediments from different oceanic and coastal regions and varying water depths. The concentration of these 3 components as a whole (biopolymeric carbon) was found to be significantly correlated (r = 0.84) with the total organic carbon concentration, suggesting that the biopolymeric fraction is representative of the total organic carbon pool. However, the system- atic variation of the biopolymeric fraction was higher than that of total organic carbon concentrations, suggesting that biopolymeric carbon is a more sensitive proxy of benthic trophic status than is the total carbon pool. Furthermore, biopolymeric carbon was significantly correlated to the amount of phytopigments, indicating that biopolymeric carbon accumulation in the sediment is related to inputs of algal carbon. Biopolymeric carbon concentrations were also positively correlated to the sediment community oxygen consumption, suggesting that the progressive accumulation of biopolymeric car- bon could be an additional co-factor potentially responsible for hypoxic or anoxic events. The enzy- matically digestible and algal fractions of biopolymeric carbon decreased in sediments with increas- ing biopolymeric carbon content (i.e. eutrophic systems), suggesting that organic carbon in eutrophic sediments is mostly refractory in nature. We propose that a biopolymeric carbon concentration in the sediment of >2.5 mg C g -1 , being associated with a bioavailable fraction of <10%, can be considered as a threshold level at which benthic consumers may experience mostly refractory organic carbon.

/pdf/quantity-and-bioavailability-of-sediment-organic-matter-as-1o3hz52pdy.pdf

Quantity and bioavailability of sediment organic matter as signatures of benthic trophic status

Benthic foraminifera as bio-indicators of trace element pollution in the heavily contaminated Santa Gilla lagoon (Cagliari, Italy).

Coastal lagoons are highly variable and dynamic systems that have been rarely investigated in terms of benthic microbial loop. We present here the results of a comparative study aimed at investigating factors and benthic processes potentially affecting microbial loop functioning in three lagoon systems (Goro, Lesina and Marsala lagoons). The three lagoons were characterised by different geo-morphological, trophic and ecological features. We determined organic matter quantity and biochemical composition, exo-enzymatic activities, bacterial biomass and bacterial carbon production together with heterotrophic nanoflagellate and meiofauna biomass. These variables allowed providing estimates of biomass ratios between different benthic compartments and gathering information on C transfer in different systems. The results of this study indicate that organic matter composition played a primary role on microbial loop ability of channelling C-biomass to higher trophic levels. In coastal lagoons, indeed, quantity and quality of sediment organic matter control rates of organic matter degradation, turnover rates (through breakdown of large macromolecules) and utilisation by benthic heterotrophic organisms (bacterial C production). Exo-enzymatic activities in all lagoons investigated were generally low, when compared to coastal marine systems, and lowest rates were observed in systems characterised by refractory organic pools. Our results indicate that the autotrophic contribution to the biopolymeric carbon pools, which influences the ratio of autotrophic to heterotrophic biomass, is a key factor regulating the functional efficiency of the benthic system. The structure of the benthic microbial loop varied according to the different characteristics of the lagoons: from a classical “detritus sink” system (i.e., the Marsala lagoon, where organic matter was highly refractory), to the “source” system (Lesina lagoon, which displayed an efficient C transfer to higher trophic levels, mediated by benthic bacteria).

Benthic microbial loop functioning in coastal lagoons: a comparative approach Boucle microbienne benthique dans les lagunes côtières : approche comparative

Benthic microbial loop functioning in coastal lagoons: a comparative approach

Effects of bottom trawling on the quantity and biochemical composition of organic matter in coastal marine sediments (Thermaikos Gulf, northwestern Aegean Sea)

In order to assess the impact of natural and anthropogenic sediment resuspension on quantity, biochemical composition and bioavailability of particulate organic matter (POM), two field investigations were carried out in two shallow coastal areas of the Mediterranean Sea. In the Gulf of Lions, we investigated the impact of a storm resuspension of sediment, whereas in the Thermaikos Gulf we investigated the impact of bottom trawling. Resuspension in the Gulf of Lions determined the increase of sedimentation rates, modified the composition of the organic fraction of settling particles and decreased the labile fraction of POM, as indicated by a drop in the enzymatically hydrolysable amino acid fraction. The increase in the refractory fraction, following short-term storm-induced resuspension, increased also the contribution of glycine and decreased the contribution of aspartic acid contents to the total amino acid pools. Trawling activities in Thermaikos Gulf determined a significant increase in suspended POM concentrations and important changes in its biochemical composition. After trawling, the protein to carbohydrate ratio decreased (as a result of a major input of sedimentary carbohydrates at the water–sediment interface) and the fraction of enzymatically hydrolysable biopolymeric C decreased by ≈30%, thus reducing the bioavailability of resuspended organic particles. Results of the present study indicate that changes in suspended POM, induced by storms and trawling activities, can have similar consequences on benthic systems and on food webs. In fact, the potential benefit of increased organic particle concentration for suspension feeders, is depressed by the shift of suspended food particles towards a more refractory composition.

Carla Fiordelmondo

Papers

Benthic microbial loop functioning in coastal lagoons: a comparative approach Boucle microbienne benthique dans les lagunes côtières : approche comparative

Benthic microbial loop functioning in coastal lagoons: a comparative approach

Effects of bottom trawling on the quantity and biochemical composition of organic matter in coastal marine sediments (Thermaikos Gulf, northwestern Aegean Sea)

Impact of natural (storm) and anthropogenic (trawling) sediment resuspension on particulate organic matter in coastal environments

Exo-enzymatic activities and dissolved organic pools in relation with mucilage development in the Northern Adriatic Sea