We measured gross and net nitrogen fixation in fractionated samples (organisms >10 µm and <10 µm), and the density of Trichodesmium, during a cruise along the northeast Atlantic boundary current system and during 2 mesoscale experiments in the upwelling systems of Cape Silleiro (northwest Iberia) and Cape Ghir (northwest Africa). The density of Trichodesmium (< 0.5 trichomes l −1 except off the northwest African coast where a gross nitrogen fixation peak of 0.98 nmol N l −1 d −1 was measured. The <10 µm fraction contributed more to both gross and net nitrogen fixation than did the >10 µm fraction in most of the areas studied. The <10 µm fraction was responsible for 70 to 92% of the total nitrogen fixation in cold nutrient-rich areas. The contribution of small diazotrophs to nitrogen fixation in the upwelling sites suggests that the distribution and activity of these organisms are more widespread than previously thought.

The horizontal and vertical distribution of representatives of diazotrophic unicellular cyanobacteria was investigated in the subtropical northeast Atlantic Ocean (28.87 to 42.00uN; 9.01 to 20.02uW). Samples from stations encompassing different water conditions (from oceanic oligotrophic waters to upwelling areas and a temperature range of 13.1uC to 24.2uC) were size fractionated and analyzed for nifH by a nested polymerase chain reaction (PCR) and by tyramide signal amplification-fluorescence in situ hybridization (TSA-FISH) using probe Nitro821. In samples from the surface, mixed-layer depth, and deep chlorophyll maximum waters, most (. 50%) of the nifH recovered was from the 0.2-3 mm fraction and was consistent with TSA-FISH counts. The , 3 mm Nitro821-positive cells were more abundant than the larger cells, and the proportion of single cells was larger than that associated with particulate matter or with larger cells. Phylogenetic analysis of representative samples revealed that most of the sequences belong to diazotrophic unicellular cyanobacteria Group A (UCYN-A or Candidatus Atelocyanobacterium thalassa). N 2 fixation in the 0.2-3 mm fraction, putatively representing the activity of UCYN-A, contributed more than 50% of the total N 2 fixation. There was a positive relationship of this putative UCYN-A abundance and activity with temperature, and a negative relationship with dissolved O 2 . The dominance of these putative UCYN-A organisms in nitrate-rich upwelling filament regions suggests that the activity of this group of organisms may not be strongly controlled by the availability of fixed N.

This study provides the first extensive data on in situ rates of N2 fixation in minimally disturbed Posidonia oceanica meadows and their temporal and spatial variability along the Mallorcan coasts in the Mediterranean Sea. Benthic bell‐jar chambers were deployed underwater enclosing (1) natural P. oceanica shoots, (2) matted sediment of P. oceanica with the leaves carefully removed by clipping to assess the contribution of N2 fixation associated with leaves, and (3) adjacent unvegetated sediment to assess the contribution of N2 fixation by the sediment. In all of the chambers, N2 fixation varied seasonally, with highest rates during summer and lowest in winter. Average N2 fixation rates in chambers enclosing natural P. oceanica shoots ranged from 0.06 mg N m−2 d−1 to 1.51 mg N m−2 d−1 (0.004–0.107 mmol N m−2 d−1) in summer. Careful removal of leaves generally reduced N2 fixation rates by 16–89%. The N2 fixation rates in unvegetated sediments ranging from 0.01 mg N m−2 d−1 to 1.99 to mg N m−2 d−1 (7.1 × 10−4–0.14 mmol N m−2 d−1) have similar magnitudes in sediments with P. oceanica. This suggests that these sediments are “hot spots” for N2 fixation. Exploratory analyses of environmental factors that may influence N2 fixation rates highlight temperature and pore water total phosphorus concentrations as important factors.

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