Biomass of the total macrozoobenthos in winter at Balgzand (1970-2016)

Macrozoobenthos at Balgzand 

The unicellular algae living in the water (phytoplankton) and on the sediment (microphytobenthos) are the main food for many of the invertebrates that live in or on top of the sediment (macrozoobenthos). These macrozoobenthic invertebrates in turn form an important food source for fish and birds.

NIOZ is studying the macrozoobenthos at Balgzand, a tidal flat area east of Den Helder, since around 1970, by sampling twice per year (end of winter and end of summer) the area on 12 fixed transects and 3 sampling plots. During low tide benthos samples are taken with a corer down to 35 cm deep into the sediment and sieved in the field. Sieved samples with both living and dead material are taken to the laboratory and sorted immediately. The living benthos organisms are identified, counted and separated, if possible, into age or size classes. Of all organisms biomass (expressed as gram ash free dry mass; AFDM) is determined.

During the 45-years of this long-term programme, important changes occurred in the species spectrum. Several invasive species entered the area, such as the bivalves American razor shell (Ensis directus) and Pacific oyster (Crassostrea gigas), and the polychaete Marenzelleria viridis. Total biomass was around 20 g m-2 ashfree drymass (AFDM) in the 1970s, and roughly doubled to around 40 g m-2 AFDM during the last decade. This long-term trend, i.e. lower values in the 1970s and higher values from ~ 1980 onwards, is roughly comparable with the pattern in phytoplankton chlopohyll-a measured in the Marsdiep tidal inlet (see Newsletter March 2016). For macrozoobenthos, the increase in biomass after 2013 more or less coincides with the increasing trend in phytoplankton chl-a that started in 2012.