Slow growth of Antarctic bryozoans increases over 20 years and is anomalously high in 2003

Some organisms are particularly appropriate models for investigation of variability in time and space for given environments. The erect bryozoan Cellarinella nutti, an endemic Antarctic species, is one such organism: it is extremely abundant, occurs from the polar front to the Antarctic continental edge, and preserves a clear macroscopic environmental record in its skeleton (growth check lines). We studied variability in the growth of 93 C. nutti individuals at depths between 247 and 414 m, at sites 1, 10, 100 and >1000 km apart in the Weddell Sea, Antarctica. Trawled C. nutti varied from 5 to 23 yr in age, and nearly half the colonies had grown asexually from fragments. We measured the annual growth increments, which ranged from ~18 zooids (8.6 mg dry mass, 0.8 mg ash-free dry mass; Age 1) to ~130 zooids (43 mg dry mass, 2.48 mg ash-free dry mass; Age 20). At 9 yr (the modal age), each C. nutti individual had precipitated >182 mg CaCO3, and by 20 yr nearly 580 mg. C. nutti grows slowly compared with other erect bryozoans, even those in the Antarctic. We found colony and site to be insignificant factors in its growth, despite the large range of distances between sites, but year was highly significant. We found a non-linear increase in growth spanning the last 2 decades that has no obvious relation to El Niño Southern Oscillation (ENSO)-associated environmental fluctuations. In particular we found that growth in 2003 was greater than in any other year measured by a factor of 2. Given its abundance, longevity, ubiquity in Antarctic waters, insignificant spatial variability in growth, but distinct annual variability, C. nutti may prove to be an important species to investigate whether benthic organisms, as well as krill, salps and fur seals, are beginning to develop major climate-related changes in phenology.

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