Temporal and spatial variability of in situ nitrogen fixation activities associated with the Mediterranean seagrass Posidonia oceanica meadows

This study provides the first extensive data on in situ rates of N-2 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 N-2 fixation associated with leaves, and (3) adjacent unvegetated sediment to assess the contribution of N-2 fixation by the sediment. In all of the chambers, N-2 fixation varied seasonally, with highest rates during summer and lowest in winter. Average N-2 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 N-2 fixation rates by 16-89%. The N-2 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 x 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 N-2 fixation. Exploratory analyses of environmental factors that may influence N-2 fixation rates highlight temperature and pore water total phosphorus concentrations as important factors.