Etude du métabolisme, à l'échelle de l'écosystème, de l'herbier de Posidonia oceanica (L.) Delille en Baie de Calvi

Human population is increasing exponentially with a growing occupation of the coastal zone.
It is estimated that by 2050, about 70% of the world population will live and/or work within 100 km of
coastlines. This trend has been going on for decades and will further impact coastal ecosystems.
Furthermore, climate changes will heavily impact littoral zones and their biotopes (sea-level increase,
change of wind regime, seawater temperature increase, …). But the littoral zone is the source of about
40% of the ecosystem goods and services provided by global ecosystems to humankind. It is then
important to study the different coastal ecosystems in terms of biogeochemistry to monitor their
evolution. Understand the interdependence between these processes and the response of coastal
ecosystems to environmental and human stressors will allow to better manage and protect them.
In this context and objective, we monitored metabolic fluxes at community scale in a Posidonia
oceanica meadow in the Bay of Calvi (Corsica, France) at -10 m depth. This monitoring contributes to
the study of the biology and ecology of the posidonia meadow since the 1970s at this site by ULiège
from STARESO. This high-frequency monitoring over 12 years (2006-2018) was carried out with a
permanent mooring equipped with oxymeters. This allowed us to compute by a mass balance of O2
the gross primary production (GPP), community respiration (CR) and net community production (NCP)
over 3275 24h-cycles. GPP, CR and NCP were highly variable whatever the time scale (daily, seasonal,
year-to-year). Day-to-day and seasonal variability of metabolic fluxes follows the one of solar
irradiance. Solar irradiance, water temperature and chlorophyll a were the main variables responsible
for the inter-annual variations of metabolic fluxes. We also observed the response of the meadow to
extreme weather conditions that nowadays are extreme events (heat-wave, absence of fall-winter
storms, …) that could become the normal situation in future with climate change in the around
Mediterranean Sea. Nowadays, GPP peaks in late Spring / early Summer (May-July), when day length
is maximal and seawater temperature optimal (17°C to 20°C), the seawater temperature peaking
seasonally (~24°C) in late August.
This favorable conjunction of light and temperature for posidonia will not be maintained with
a 3°C increase of seawater temperature projected for 2100. Climate models predict that storms in
autumn and winter will decrease during the next decades. Data collected during years characterizes
by absence of fall storms and/or of mild winters with low winds showed a decrease of both GPP and
CR. We showed that this decrease was probably related to the absence of the export of litter, which
shadows the benthos, blocking the development of sciaphyle flora. Further, a large part of nutrients
stored by P. oceanica are absorbed by leaves from the water column. Mild and calm winters favor an
early stratification of the water column, re-enforcing its oligotrophic nature. The mooring of oxymeters
was an efficient and robust tool to carry out the monitoring of the P. oceanica meadow.
We made incubations with benthic chambers during the first three years of the monitoring
allowing to make the full budget of inorganic and organic carbon. The P. oceanica meadow is largely
autotrophic at annual and community scales as GPP is higher thatn -CR (NCP>0) with 83, -60 and
23 mol de C m-2 yr-1 for GPP, CR and NCP, respectively. NCP was positive for 90% of the 3275 daily
cycles. The incubations showed that a dissolution of calcium carbonate occurred in the meadow, at
a rate of 7 mol Ca CO3 m-2 yr-1. For seawater pH and total alkalinity standard conditions, this
corresponds to an additional sink of 4 mol O2 m-2 yr-1 to be added to 23 moles de CO2 m-2 yr-1 sustained
by NCP. The P. oceanica meadow at -10 m in the Bay of Calvi is a net sink of CO2 of
27 moles de CO2 m- 2 yr-1.