Coastal marine ecosystems, which play a crucial role in the biogeochemical and ecological functioning of the Earth, are highly sensitive to the combined effects of climate and human activities. ...Because of their location, coastal ecosystems are directly influenced by human activities, but it remains challenging to assess the spatial and temporal scales at which climate influences coastal ecosystems. We monitored 12 sampling stations, distributed in 8 ecosystems in France, over 2 decades for physico-biogeochemical parameters (temperature, salinity, concentrations of dissolved oxygen, nutrients and particulate material). The study encompasses a large diversity of temperate coastal ecosystems with respect to e.g. geomorphology, trophic status, tidal regime, river influence and turbidity. Time-series analysis coupled with standardised 3-mode principal component analyses, partial triadic analyses and correlations were used to assess bi-decadal variability and ecosystem trajectories, and to identify large-scale, regional and local drivers. Our results highlighted 2 abrupt changes in 2001 and 2005. The bi-decadal changes were related to changes in large-scale and regional climate, detected through proxies of temperature and atmospheric circulation, as well as through river discharge. Ecosystem trajectories tended to move towards an increase in temperature and salinity, and/or a decrease in chlorophyll
a
, nutrients and particulate matter. However, the magnitude of change, the year-to-year variability and the sensitivity to the 2001 and 2005 changes varied among the ecosystems. This study highlights the need for establishing long-term time series and combining data sets as well as undertaking multi-ecosystem and local studies to better understand the long-term variability of coastal ecosystems and its associated drivers.
Nitrogen uptake by net- (15–200 μm), nano- (1–15 μm) and picoplankton (<1 μm) was measured over seasonal cycles at two stations with different patterns of biological and chemical cycles in the ...Morlaix Bay (western English Channel). Though assimilable dissolved N nutrient pool at both stations was nitrate-dominated, characteristics of biomass and N uptake by netplankton differed from conventional patterns in two respects. In the first, biomass (26–30%) and N uptake (36–43%) were less important than those of nanoplankton. In the second, the netplankton did not show any marked preference for nitrate over ammonium (nitrate to ammonium uptake ratios of 0.98 and 1.08). In contrast, nanoplankton had a preference for ammonium over nitrate (ammonium to nitrate uptake ratios of 2 and 1.2). N uptake by picoplankton was only 8% of total N uptake at both stations and was supported mainly by regenerated N (66% ammonium and 17% urea), with nitrate uptake detectable in only one instance and nitrite uptake in none. Substrate-dependent uptake of ammonium in all fractions and a higher ammonium uptake in the nanoplankton fraction in summer at both stations when ambient ammonium concentrations were high indicated that while nitrate may satisfy a part of N requirements, availability of ammonium and its flux through nanoplankton determine the magnitude of total N uptake in these waters. Most of the N uptake in picoplankton appears to be autotrophic, suggesting that a substantial part of heterotrophic uptake, if any, could be localized in the fractions >1 μm, and mediated by free-living and particle-bound bacteria.
From 2008 to 2014, the MAREL-Iroise buoy, located in the Bay of Brest, collected high-frequency measurements of partial pressure of CO2 (pCO2) and ancillary hydrographic parameters, in conjunction ...with a comprehensive sampling regime of two additional carbonate system variables total alkalinity (AT), and dissolved inorganic carbon (DIC). Biological processes drive variations in AT and DIC throughout the year, except in winter, when primary production is negligible and large freshwater inputs occur. Annually, the Bay of Brest generally behaves as a source of CO2 to the atmosphere (0.14±0.20molCm−2yr−1), showing inter-annual variability significantly linked to annual net community production (NCP). The presence of a large community of benthic filter feeders leads to high levels of particulate organic matter (POM) and opal deposition during the spring diatom bloom. Over the following few months, benthic POM remineralisation reduces the spring CO2 deficit relative to the atmosphere, and remineralisation of biogenic silica supplies further late spring primary production. The result is an inverse spring NCP – air-sea CO2 flux relationship, whereby greater NCP in early spring results in lower fluxes of CO2 into the Bay in late spring. This recycling mechanism, or silicic acid pump, also links the spring and summer NCP values, which are both determined by the peak wintertime nutrient concentrations. The carbonate system is further affected by the benthic community in winter, when CaCO3 dissolution is evident from notable deviations in the ΔAT:ΔDIC ratio. This study highlights the necessity of individual study of coastal, temperate ecosystems and contributes to a better understanding of what determines coastal areas as sinks or sources of CO2 to the atmosphere.
•The Bay of Brest generally behaves as a source of CO2 to the atmosphere.•Inter-annual variability in air-sea CO2 exchange is linked to net community production.•Springtime net community production is determined by the winter, river dissolved silica supply.•Total alkalinity and dissolved inorganic carbon variability is driven by biology.
Measurements of ammonium and nitrate uptake by a Gyrodinium bloom in the Ushant frontal region showed that ammonium is the preferred form and its assimilation provides 90% of the N required. Uptake ...(3.75 mmol m-2 h-1) and regeneration (3.41 mmol m-2 h-1) of ammonium were tightly coupled. Regenerative flux to ammonium was throught a microbial loop of bacteria, nanoflagellates, and ciliates in the size fraction$<30 \mu m$. Comparison of the conditions before and after the bloom formation, and a mass balance of N, shows that the G. aurelom bloom in the frontal region can be maintained almost exclusively by in situ remineralization of remnants of previous diatom blooms.
Examination of data from literature showed that non-inclusion of urea uptake in nitrogen uptake by phytoplankton results in statistically significant overestimates off. The extent to which f was ...overestimated because of non-inclusion of urea uptake depended upon the ecosystem type and was 6, 17, 24, 42 and 55%,, respectively, in upwelling, coastal, polar, oceanic and estuarine waters. Systematic urea uptake measurements thus become necessary, but where such data are lacking, corrections are possible with the ratio between the f values calculated with and without urea uptake for each ecosystem.
Ammonium assimilation and regeneration by size-fractionated plankton were measured for 1 year at a coastal station in the permanently well-mixed waters of the western English Channel. The lowest ...assimilation and regeneration rates (<1 ng-at N l−1 h−1) were in winter and the highest (>25 ng-at N l−1 h−1) in summer. Vertical profiles showed a light-dependent pattern in assimilation and regeneration, with the maximum rates at intermediate depths and the lowest at the base of the euphotic zone. Nanoplankton (1–15 µm) assimilated and regenerated more ammonium throughout the year than net-(15–200 µm) and picoplanlcton (<1 µm). Assimilation in net- and nanoplankton was regulated by changes in biomass rather than by photosynthetic efficiency. Assimilation in picoplankton was mainly bacterial, but the autotrophic contribution to it became substantial in spring-summer. Ciliates and bacteria were more important for ammonium regeneration than flagellates. Assimilation to regeneration ratios varied as a function of size class: from 1 in picoplankton through 1.8 in nanoplankton to 2.4 in netplankton. Ammonium regenerated in the whole water column exceeded assimilation requirements in the euphotic zone and this may explain the accumulation of ammonium in spring-summer observed in these waters.
PDF
