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The Mediterranean region under climate change

 | 
Jean-Paul Moatti
, 
Stéphane Thiébault

Part 2. Vulnerability and impacts

Sub-chapter 2.1.1. Climate change impact on planktonic production in the Mediterranean Sea

Fabio Benedetti

Texte intégral

Primary production and plankton communities in the Mediterranean Sea

1The Mediterranean Sea is characterized by a gradient of growing oligotrophy from the northwestern regions to the Levantine basin (Bosc et al. 2004). Nutrient availability is generally low, resulting in low phytoplankton biomass (less than 0.25 mg/mL). However, blooms and peaks in zooplankton biomass are recorded in areas of complex physical dynamics (winter convection, fronts, and gyres, Siokou-Frangou et al. 2010).

2Seasonal hydrological regimes structure the phytoplankton community (Marty et al. 2002; Marty and Chiavérini 2010). Seasonal patterns are linked to variations in hydrological features and changes in nutrient concentrations: peaks in biomass occur in spring (March-April), after the replenishment of nutrients in surface waters owing to winter water mixing. After spring, the phytoplankton biomass decreases and goes deeper in the water column. Relying on satellite observations, D’Ortenzio and Ribera d’Alcalà (2009) defined a bioregionalization of the whole basin. Their classification provided a clear picture of the different phytoplankton trophic regimes in the Mediterranean. Nearly 60% of the basin’s surface was described as «non-blooming» (mainly the eastern regions and the southwestern basin). Conversely, the northernmost regions, the Alboran, and the region of Rhodes were shown to be zones of «intermittent blooming» (gradual growth of biomass between September and February, linked to the deepening of the mixed layer). The locations of the different trophic regimes are clearly linked to convection and deep water formation processes (Millot and Taupier-Letage 2005). Primarily wind driven physical forcings favor the development of the phytoplankton by re-injecting nutrients (nitrates and phosphates) into the surface waters through mixing of the water column (Sverdrup 1953). This has a strong impact on the structure of the phytoplankton community.

3Depending on the trophic regime, different phytoplankton taxa are likely to constitute the communities. Marty et al. (2002) analyzed algal pigment content to explore the relative contribution of different taxa to total phytoplankton biomass. These authors found that spring blooms were dominated by diatoms (large micro-algae characterized by silica shells), while stratifying conditions favor the development of nanoflagellates (nanophytoplankton), which are then replaced by cyanobacteria (picophytoplankton). The two latter phytoplankton size classes can be considered as markers of oligotrophic conditions, while microphytoplankton (diatoms) are opportunistic and burst after nutrient replenishment. Over the year, nanophytoplankton account for 43% to 50% of the total primary production of the Mediterranean Sea, which is largely dominated by Prymnesiophyta (Uitz et al. 2012). Even the blooming areas investigated by D’Ortenzio and Ribera d’Alcalà (2009) were dominated by nanophytoplankton, except in spring when they were replaced by microphytoplankton (up to 38% of total primary production). In non-blooming zones, picophytoplankton come after nanophytoplankton in order of abundance.

4To date, offshore surveys of plankton distribution and communities have been dispersed not only in space and time, but have also used different methods (Siokou-Frangou et al. 2010). Consequently, regional and seasonal patterns of diversity and community composition are still poorly understood.

5Concerning phytoplankton diversity, low biomass is linked to the dominance of the smallest plankton, which consist of picophytoplankton (mainly prochlorophytes), cyanobacteria (Synechococcus) and flagellates (Marty et al. 2002; Uitz et al. 2012). Non-colonial picodiatoms have occasionally been reported to be abundant, but cell size usually prevents their accurate identification. Nanophytoplankton, which are mainly composed of small flagellates, dinoflagellates, coccolithophores, and to a lesser extent of some small solitary diatoms, are also very abundant. Major increases in biomass are correlated with the growth of microphytoplankton, which are mainly composed of large colonies of diatoms (Marty et al. 2002; Marty and Chiavérini 2010). The main genera are Asterionellopsis, Chaetoceros, Pseudo-nitzschia, Thalassionema and Thalassiosira (Siokou-Frangou et al. 2010). These genera are unevenly distributed in the Mediterranean basin: healthy colonies of Chaetoceros and Pseudo-nitzschia have been observed in areas of deep convection while colonies of Chaetoceros, in association with Thalassiosira, Proboscia, Rhizosolenia and Leptocylindrus have been found across fronts and gyres (circular oceanic surface currents). Although among the less abundant microplankton, dinoflagellates show significant diversity in the Mediterranean; like nanoplankton, they are associated with stratified and nutrient-depleted conditions. These taxa exhibit wide trophic modes: some are heterotrophic, while others are mixotrophic (Neoceratium spp.) or host endosymbiotic cyanobacteria. To summarize, Mediterranean phytoplankton comprise very diverse taxa with diverse ecological preferences.

6Regarding zooplankton, the world dominance of copepods in the water column also applies to the Mediterranean. Like micro-algae, smaller species (< 2 mm) prevail in the communities, whatever the trophic regime (Siokou-Frangou et al. 2010). The bulk of zooplankton comprises very diverse genera of calanoids and cyclopoids. The relative contribution of the smaller cyclopoids is thought to increase with the increasing west-east gradient of oligotrophy, while larger species are more abundant in colder and more productive areas (Siokou-Frangou et al. 1997).

7Other zooplankton taxa should not be overlooked. Cladocerans are found in large numbers in summer in coastal environments (Riandey et al., 2005). Gelatinous filter-feeders, like salps, frequently produce spectacular blooms in warm waters. Outbursts of jellyfish in summer have become a public concern. Gelatinous-wise, chaetognaths and siphonophores are carnivorous species frequently encountered in the Mediterranean Sea that prey on copepods and other smaller planktonic organisms.

Climate variability influences plankton distribution and community composition

8Plankton abundance and distribution are strongly controlled by hydrological features and water mass advection. The tight coupling between their population dynamics and climate makes them optimal indicators to monitor the impact of climate variability on ecosystems (Hays et al. 2005). Tunin-Ley et al. (2009) assembled time series to investigate the effects of increasing temperatures on the distribution of 46 Ceratium species (Dinoflagellates) over the 20th century (1908-2005). Irrespective of the location, species composition showed a clear seasonal cycle, but phenologies differed according to the species and the sites surveyed. Although Ceratium assemblages did vary with changing temperatures, contrary to expectations, thermophilic species did not show increasing trends. As no new species were detected during the 20th century, the disappearance of species that prefer colder conditions could not be balanced, and warming has resulted in a loss of biodiversity.

9Marty and Chiavérini (2010) monitored hydrological changes in the Ligurian basin and their biogeochemical consequences during the period 1995-2007. These authors revealed an increase in phytoplankton biomass (+1.5 mgChla/m2. yr) paralleling increases in temperature and salinity in the northwestern Mediterranean Sea. Furthermore, the fraction of biomass attributed to diatoms also increased. Thus, the increases in biomass were due to generalized growth of phytoplankton, not only to the growth of the smaller size classes that usually dominate in warmer conditions. Marty and Chiavérini (2010) reported an increase in the frequency of mixing events, rather than longer stratification periods.

10Long time series provide ideal material to study plankton dynamics under climate change. Several multidecadal surveys have been conducted in the Mediterranean Sea (Berline et al. 2012), all of which evidenced strong seasonal patterns in phytoplankton and zooplankton communities (Ribera d’Alcalà et al. 2004). In the Gulf of Naples for instance, a time series from 1984 to 2000 revealed an over decadal decrease in phytoplankton (Ribera d’Alcalà et al. 2004), together with an increase in a rare copepod species and a weak decrease in zooplankton biomass. These authors suggested that biological rhythms regulate the temporal dynamics while the climate modulates the amplitude of the growth periods. This hypothesis was later invalidated by Mazzocchi et al. (2011) using the same time series, by providing evidence for phenological changes in Mediterranean mesozooplankton: the copepod assemblage characterizing spring and summer conditions declined over the study period as its typical high production season became shorter and started earlier in the year. By contrast, the communities that depend on summer and fall conditions remained stable phenology-wise, and were quite resilient to decreases in phytoplankton biomass. From these results, Mazzocchi et al. (2011) concluded that the zooplankton composition does not vary according to phytoplankton variability, and that zooplankton communities are resilient to climate change. This conclusion was challenged by Conversi et al. (2009), who demonstrated a dramatic shift across the copepod community between the late 1980s and the early 1990s. Warming combined with changes in circulation in the Ionian Sea (Civitarese et al. 2010) have been identified as the principal causes. The link between climatic variability and phytoplankton dynamics is consequently not well understood yet.

11To summarize, plankton assemblages in the Mediterranean have proved to be good indicators of environmental changes. The variety of trophic regimes and the diversity of biological communities offer great opportunities to test hypotheses concerning climate change and its impacts on plankton. The Mediterranean Sea contains a wide range of plankton whose environmental preferences and functional roles remain to be fully determined. The patterns described above suggest differences between the ecological traits and environmental preferences of species. By altering the pelagic environment, anthropogenic climate change could alter plankton distribution, as well as their importance in the food web.

How will anthropogenic climate change impact Mediterranean plankton?

12Despite the importance of plankton for both food webs and biogeochemical cycles, very few studies have attempted to forecast the impact of climate change on primary production and community composition in the Mediterranean Sea (Lazzari et al. 2013; Herrmann et al. 2014). There are projections at the global scale, but the corresponding models do not adequately resolve the peculiar regional processes of the Mediterranean, and regional coupled models thus need to be implemented.

13Focusing on the northwestern areas of the basin (Gulf of Lion and Ligurian Sea), Herrmann et al. (2014) investigated the response of the pelagic plankton ecosystem and associated carbon cycle, to long term changes in oceanic and atmospheric circulation. Their predictions suggest that climate change will not modify the seasonal dynamics and variability of the planktonic ecosystem at a first order, compared to what is modeled for the contemporary period. Microphytoplankton and nanophytoplankton biomasses are not expected to increase by the end of the 21st century. Meanwhile, their model forecasts an increase in zooplankton biomass, due to a gain in the smaller size fractions (nanozooplankton), and in picophytoplankton biomass. Their study suggests that climate change in the northwestern Mediterranean will favour the smallest components of the plankton, and strengthen the microbial loop activity.

14Lazzari et al. (2013) used a different model that covered the entire Mediterranean Sea to assess the impact of climate change on the carbon cycle in a plankton ecosystem model. Like Herrmann et al. (2014), their model predicts a strengthening of the microbial pathway in the plankton ecosystem with reduced nutrient availability and phytoplankton biomass.

15In contrast to higher trophic levels (Ben Rais Lasram 2010; Albouy et al. 2012), the potential impact of rising temperatures and salinity on plankton species composition has never been assessed. This will be crucial to better understand how climate change could reshape plankton in the Mediterranean Sea, as not all species share similar traits and functions in the ecosystems (Benedetti et al. 2016). In the future, more modeling studies are needed to better constrain the predicted impacts of climate change on primary production and plankton size structure. Models should also focus on resolving taxonomic and ecological complexity by accounting for differences in plankton species traits. Finally, multi-trophic models should be developed to estimate how climate change may impact the structure of the food web and ecosystem services provision.

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Auteur

UPMC, CNRS, UMR LOV, France
Marine ecologist, Sorbonne Universités, Université Pierre et Marie Curie (UPMC), Université Paris 06, Laboratoire d’Océanographie de Villefranche (LOV), Observatoire Océanologique, Villefranche-sur-Mer, France
fabio.benedetti24@gmail.com

© IRD Éditions, 2016

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