Abundance of phytoplankton, heterotrophic nanoflagellates and bacteria through the water column at time series station L4 in the Western English Channel. 2007-2011

The marine laboratories in Plymouth have sampled at two principle sites in the Western English Channel for over a century in open-shelf (station E1; 50° 02'N, 4° 22'W) and coastal (station L4; 50° 15'N, 4° 13'W) waters. These stations are seasonally stratified from late-April until September, and the variable biological response is regulated by subtle variations in temperature, light, nutrients and meteorology. Station L4 is characterized by summer nutrient depletion, although intense summer precipitation, increasing riverine input to the system, results in pulses of increased nitrate concentration and surface freshening. The winter nutrient concentrations at E1 are consistent with an open-shelf site. Both stations have a spring and autumn phytoplankton bloom; at station E1, the autumn bloom tends to dominate in terms of chlorophyll concentration. The last two decades have seen a warming of around 0.6°C per decade, and this is superimposed on several periods of warming and cooling over the past century. In general, over the Western English Channel domain, the end of the 20th century was around 0.5°C warmer than the first half of the century. The warming magnitude and trend is consistent with other stations across the north-west European Shelf and occurred during a period of reduced wind stress and increased levels of insolation (+20%); these are both correlated with the larger scale climatic forcing of the North Atlantic Oscillation.

Nitrification rates in the NW Mediterranean Sea

During spring, ammonium oxidation and nitrite oxidation rates were measured in the NW basin of the Mediterranean Sea, from mesotrophic sites (Ligurian Sea and Gulf of Lions) to oligotrophic sites (Balearic Islands). Nitrification rates (average values for 37 measurements) ranged from 72 to 144 nmol of N oxidised/l/d, except in the Rhône River plume area where the rates increased to 264-504 nmol/l/d because of the riverine inputs of nitrogen. Maximal rates were located around the peak of nitrite within the nitracline at about 40 to 60 m and just above the phosphacline. At 1 station, relatively high values of nitrification (50 to 130 nmol/l/d) were also measured deep in the water column (240 m). Day-to-day variations were measured demonstrating the response within a few hours to hydrological stress (wind-induced mixing of the water column) and showing the role of hydrological characteristics on the distribution of nitrification rates. Because of the homogenous temperature (13°C) in the Mediterranean Sea, the spatial (geographical and vertical) fluctuations of nitrifying rates were linked to the presence of substrate due to mineralisation processes and/or Rhône River inputs. We estimate the contribution of nitrate produced by nitrification to the N demand of phytoplankton to range from 16% at mesotrophic to 61% at oligotrophic stations.