Long-term and large-scale variability in productivity of the tellinid bivalve Macoma balthica on Wadden Sea tidal flats

Abstract

A series of 27 successive annual estimates of production of soft parts of the bivalve Macoma balthica (L.) populating the tidal flats of Balgzand (western Wadden Sea, The Netherlands) was obtained using multi-station (15 sites) and long-term (27 yr: 1978 to 2004, inclusive) monitoring data on numerical densities and ash-free dry mass (AFDM). Annual production values were esti- mated from seasonal growth increments, which were positive in spring and early summer and (in adults) negative in the remainder of the year, when up to 70% of individual weight gain of the preceding growing season was lost. Annual estimates based on twice-annual sampling were checked and refined on the basis of a detailed 2 yr study. Weight loss proportions increased with age of ani- mals and with water temperature in winter. Net annual somatic production of the entire population varied from 1 to 9 (27 yr mean: 2.9 ± 0.3) g AFDM m -2 yr -1 . Lifetime production per individual averaged 0.007 g AFDM for recruits of ~3 mo old and 0.032 g AFDM for ~10 mo olds. Numerical den- sities of ~10 mo old recruits predicted, within narrow limits, subsequent cohort production by way of a positive linear relationship. However, production of 3 mo old recruits was on average higher at low than at high densities, owing to better juvenile survival at low densities. Local (between-station) variation in mean annual production was primarily related to local variation in recruitment. However, production per recruit was significantly higher at low than at high intertidal levels, in accordance with higher growth and lower mortality rates at lower levels. Strong temporal and local variation was also found in annual production/biomass (P/ - B) ratio values, which reflected the variable age compo- sition of the population: P/ - B ratio values were augmented whenever mean age was low. Consistently higher mean P/ - B ratio values (~0.6 vs. ~0.4 yr -1 ) were found at high near-coast compared with low off-shore tidal flats, in accordance with higher mortality rates on high flats. Environmental factors affecting variation in annual production included between-year variability in recruitment (which was related to winter temperature and shrimp predation pressure) and growth (which was related to diatom abundance).