Short-term response of benthic bacteria and nanoflagellates to sediment resuspension: An experimental study

We investigated benthic bacterial abundance, biomass and activities and heterotrophic nanoflagellate (HNF) abundance and biomass during sediment resuspension and after a resilience period through short-term laboratory experiments. The strong sediment resuspension increased bacterial carbon production only slightly (Tukey's test, P < 0.05) but somehow interfered with microbial growth. During sediment resuspension, after an initial increase (at 48 and 72 h) bacterial abundance and biomass decreased until the end of resuspension. Sediment resuspension also altered the relationship between HNFs and bacteria (increase in the ratios of bacterium to HNF abundance and biomass after sediment resuspension; Tukey's test, P < 0.01). After sediment resuspension, the fraction of bacterial C production removed by flagellates decreased, whereas bacterial turnover did not change significantly. suggesting that HNF pressure on bacteria decreased. These results are consistent with the significant decrease in both HNF abundance and biomass (Tukey's test, P < 0.01) and suggest that this small benthic component is less resilient to sediment resuspension than bacteria. Sediment resuspension also determined significant changes in sediment organic matter pools (proteins and carbohydrates). Sediment resuspension determined a decrease in total protein concentrations, whereas total carbohydrate concentrations did not change significantly. Sediment resuspension had significant effects on extracellular enzymatic activities, as aminopeptidase increased and β-glucosidase activity decreased. Based on total protein and carbohydrate concentrations and enzymatic activities, we observed that sediment resuspension resulted in an increase in protein turnover and a decrease in carbohydrate turnover. These results indicate that sediment resuspension plays a major role in the early diagenesis of sediment organic matter

Short-term impact of clam harvesting on sediment chemistry, benthic microbes and meiofauna in the Goro lagoon (Italy)

The effects of clam harvesting on sediment organic matter, bacteria, heterotrophic nanoflagellates, ciliates and meiofauna were investigated through a short-term experiment carried out in the Goro lagoon during summer 2000. Sediment samples were collected in two areas: an impacted and an undisturbed (both 25 m2), before and after (5, 24 and 48 h) clam harvesting. Immediately after sediment reworking, total organic matter content decreased for about 20%. Different organic compounds displayed different redeposition patterns resulting in the alteration of the biochemical composition of sediment organic matter. Bacterial abundance decreased significantly after clam harvesting, but microbial enzymatic activities and bacterial C production were relatively unaffected. Heterotrophic nanoflagellates displayed a positive response to sediment disturbance, whereas ciliate and meiofaunal abundance did not display any significant response. Results of this short-term field experiment suggest that clam harvesting determines a positive impact on nutritional availability of sediment organic matter, thus facilitating organic carbon transfer to higher trophic levels of the benthic microbial loop

Benthic microbial loop functioning in coastal lagoons: a comparative approach

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). (C) 2003 Editions scientifiques et medicales Elsevier SAS and Ifremer/CNRS/IRD. All rights reserved.Très peu d'études existent sur la boucle microbienne dans les lagunes côtières, systèmes dynamiques à grande variabilité. Nous présentons ici les résultats d'une étude comparée s'intéressant aux facteurs du milieu et aux processus benthiques qui affectent le fonctionnement du réseau microbien dans trois systèmes lagunaires (Goro, Lesina et Marsala). Ces trois lagunes présentent des aspects géomorphologiques, trophiques et écologiques différents. Nous avons mesuré la teneur en matière organique et la composition biochimique, les activités exo-enzymatiques, la biomasse bactérienne et la production bactérienne carbonée ainsi que la biomasse de la méiofaune et des flagellés hétérotrophiques. Nous avons ainsi pu estimer les taux de transfert de biomasse et donc les flux de carbone entre les différents compartiments benthiques. La composition en matière organique joue un rôle primordial sur l'aptitude de la boucle microbienne à canaliser la biomasse de carbone vers les échelons supérieurs. Dans les lagunes côtières, la qualité et la quantité de matière organique sédimentaire contrôlent le taux de dégradation de la matière organique, les taux de renouvellement (par destructuration des grosses molécules) et l'utilisation par les hétérotrophes benthiques (production carbonée bactérienne). Dans les trois lagunes étudiées, les activités exo-enzymatiques sont faibles comparé aux écosystèmes marins côtiers ; les taux les plus bas se rencontrent dans les systèmes caractérisés par un pool organique réfractaire. Aussi, les taux de mobilisation de la matière organique sont-ils bas. Ces résultats montrent que la contribution autotrophique aux pools de carbone des biopolymères qui influencent le rapport biomasse autotrophe / biomasse hétérotrophe constitue un facteur de régulation essentiel de l'efficience de la boucle microbienne. La structure de ce réseau microbien varie d'une lagune à l'autre entre deux situations opposées : d'un classique « puits à détritus » (par exemple la lagune de Marsala où la matière organique est hautement réfractaire) à un système « source » (la lagune de Lesina avec un transfert efficace du carbone vers les niveaux trophiques supérieurs par l'intermédiaire des bactéries benthiques)

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

The effects of bottom trawling on quantity and the biochemical composition of sediment organic matter were assessed, along a gradient of trophic state in a coastal area of the Gulf of Thermaikos (Aegean Sea). Total organic carbon concentrations and other organic variables (such as biopolymeric C concentrations) displayed a significant increase immediately after the initiation of trawling activities, reaching unexpectedly high values when compared to a control (i.e. samples collected before trawling events). The lack of significant changes in photopigment content of the sediments, observed after trawling events, indicates that primary production processes did not change markedly. Since also sediment properties did not vary significantly during the period of investigation, the observed changes in sediment organic content can be related with organic matter uplift from deeper sediment layers, caused by reworking (bottom up transfer) and/or reduced C and N consumption by benthic biota. Trawling determined also significant changes in the biochemical composition of sediment organic matter. Indeed, sediment resuspension increased the hydrolysable fractions of protein and carbohydrate pools, likely to be the result of increased degradation rates under oxic conditions. Such an effect indicates that trawling may increase the quality and bioavailability of organic C to consumers; this, in turn, could modify the energetic and trophic state of the benthic systems. We conclude that bottom trawling might have important trophodynamic consequences, for benthic microbial and meiofaunal assemblages

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

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

Benthic microbial abundance, enzymatic activities, bacterial production, C mineralization and oxygen consumption rates in a highly trawled ecosystem (Thermaikos Gulf, Aegean Sea)

Abundance of benthic bacteria, heterotrophic nanoflagellates and ciliates, extracellular enzymatic activities, bacterial C production, C mineralisation and sediment community oxygen consumption rates were measured in the Thermaikos Gulf (Northeastern Mediterranean), before (September 2001), and during intense trawling activities (October 2001 and February 2002). The biochemical composition of sedimentary organic matter has revealed that bottom trawling had an effect on the trophic state of Thermaikos Gulf. Changes on the benthic microbial food web were also recorded, during the three sampling seasons. Even though trawling-induced sediment resuspension did not alter significantly the abundance of the microbial components, with the exception of the most impacted station, it determined changes regarding their relative importance. Thus, the ratios of bacterium to nanoflagellates and ciliate to nanoflagellates abundance increased in the trawled stations, causing a sudden increase in bacterial C production, in comparison to the non-trawled station. Four months later, the effects of trawling on the microbial food web were less evident, masked possibly by the drastic decrease in the water temperature. The results of the present work suggest that bottom trawling induces alteration of the sedimentological variables and can be considered as a factor affecting the function of the microbial food web in marine coastal ecosystems. These alterations cause faster mobilisation of organic C buried in the sediment and increase nutrient concentrations and availability in the system, thus inducing an effect that could lead to coastal eutrophication