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

Abstract

Abstract 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 × 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.