Plankton food-web functioning in anthropogenically impacted coastal waters (SW Mediterranean Sea): An ecological network analysis

Abstract

The study is the first attempt to (i) model spring food webs in three SW Mediterranean ecosystems which are under different anthropogenic pressures and (ii) to project the consequence of this stress on their function. Linear inverse models were built using the Monte Carlo method coupled with Markov Chains to characterize the food-web status of the Lagoon, the Channel (inshore waters under high eutrophication and chemical contamination) and the Bay of Bizerte (offshore waters under less anthropogenic pressure). Ecological network analysis was used for the description of structural and functional properties of each food web and for inter-ecosystem comparisons. Our results showed that more carbon was produced by phytoplankton in the inshore waters (966-1234 mg C m-2 d-1) compared to the Bay (727 mg C m-2 d-1). The total ecosystem carbon inputs into the three food webs was supported by high primary production, which was mainly due to >10 µm algae. However, the three carbon pathways were characterized by low detritivory and a high herbivory which was mainly assigned to protozooplankton. This latter was efficient in channelling biogenic carbon. In the Lagoon and the Channel, foods webs acted almost as a multivorous structure with a tendency towards herbivorous one, whereas in the Bay the herbivorous pathway was more dominant. Ecological indices revealed that the Lagoon and the Channel food webs/systems had high total system throughput and thus were more active than the Bay. The Bay food web, which had a high relative ascendency value, was more organized and specialized. This inter-ecosystem difference could be due to the varying levels of anthropogenic impact among sites. Indeed, the low value of Finn's cycling index indicated that the three systems are disturbed, but the Lagoon and the Channel, with low average path lengths, appeared to be Highlights ► Herbivorous and multivorous food-webs were the dominant carbon pathways in the Bay and the Lagoon/Channel, respectively during the study period. ► Indices used, showed that the Lagoon and the Channel seemed to be more stressed, more active, and less organized compared to the Bay of Bizerte. ► Inverse modelling combined with ecological networks analysis may offer an effective tool for management and assessment of ecosystems health, and to diagnose the occurrence of anthropogenic pressures.

Seasonal changes of chemical contamination in coastal waters during sediment resuspension

“…Similar seasonal and spatial patterns were also observed for plankton (bacterioplankton, phytoplankton and zooplankton) (Meddeb et al, 2018;Salhi et al, 2018) as well as in comparable coastal ecosystems (Aubry and Acri, 2004) suggesting close interactions between the planktonic communities and the chemical composition of the marine water Pringault et al, 2016).…”

A new type of plankton food web functioning in coastal waters revealed by coupling Monte Carlo Markov chain linear inverse method and ecological network analysis

“…Seasonal dynamics has previously been shown for phytoplankton, with high chlorophyll biomass (2-4 µg l -1 ) during spring and summer, and lower values in the other seasons (<1.5 µg l -1 ). This coincided with the predominance of diatoms (10 5 cells l -1 ) in spring and summer, and of nanoflagellates during the rest of the year (Sahraoui et al, 2009, Meddeb et al, 2018. Diatoms are also present in autumn and winter and may contribute up to 20-35% to the phytoplankton assemblage (Sahraoui et al, 2009).…”

“…met1 and met2) and zooplankton sloppy feeding. Protozooplankton grazed on bac, pic and nan, but also on mic, since they have been reported as potential grazers on large preys in several coastal areas, including our study sites (Meddeb et al, 2018). Metazooplankton, known as inefficient grazers on particles < 5 µm (Fortier et al, 1994), consumed nano-and microphytoplankton, but not picophytoplankton.…”

“…Imposing linear inequalities for each unknown flow helps to reduce the range of possible solutions. Two ranges of inequalities were adopted, as previously considered in Meddeb et al (2018). Firstly, a bound of an upper and lower values for each flow were considered by the calculation of a confidence interval around the field data, i.e.…”

“…Up to date, the food web of Bizerte Lagoon has been modeled during summer (Grami et al, 2008), showing an efficient transfer of primary production through the herbivory of microzooplankton. Also, the food webs of the Bay, Channel and Lagoon of Bizerte has been modelled during spring (Meddeb et al, 2018). However, these studies were based on data collected during one season only, and there exist little knowledge about the seasonal variability in food web structures and dynamics in this region and how they fit with the typology of plankton functioning.…”