Microphytobenthic communities of subtidal locations in New Zealand: taxonomy, biomass, production, and food-web implications

Diatom mats dominated by large sigmoid, raphid species of the genus Gyrosig ma (Naviculaceae) and, in some instances, the tube-former, Berkeleya rutila ns (Naviculaceae) were observed in the semi-enclosed coastal waters (depths 6-20 m) of Tory Channel, Marlborough Sounds, New Zealand. The latter speci es, not previously observed in New Zealand, is generally associated with in tertidal environments where light intensities are relatively higher than su btidal locations. Its presence imparted a filamentous nature to the mat tha t may play a role in maintaining the integrity of the sediment/water interf ace. Sediment chlorophyll a concentrations ranged from c. 20 to 200 mg m(-2 ). Microphytobenthic (MPB) contributions to the total (benthic + planktonic ) chlorophyll a equated to 92, 89, and 84% at depths of 8, 16, and 20 m, re spectively. Oxygen production in intact cores often exceeded consumption (e .g., by up to 2.5 mmol m(-2) h(-1)) under near-in situ conditions of light (8-30 mu mol m(-2) s(-1)). The feeding strategies of major components of th e benthic macroinvertebrate fauna were used to identify possible pathways f or utilisation of MPB production. MPB communities of more exposed subtidal locations in Tasman Bay were generally less dense and comprised of differen t species dominated by Pleurosigma amara and/or Paralia marina. We conclude that microalgal communities inhabiting the soft sediments in these regions can provide an important contribution to the benthic food web.