Unprecedented slow growth and mortality of the rare colonial cyanobacterium, Nostoc zetterstedtii, in oligotrophic lakes

Abstract

Centimeter‐large colonies of Nostoc zetterstedtii from a Swedish oligotrophic lake had the lowest growth and mortality rates of any studied temperate macrophyte. Annual growth rates at two shallow sites averaged 0.57‐0.73 × 10−3 d−1, corresponding to doubling times of colony dry weight in 2.6‐3.3 yr. With such low growth rates, the largest colonies measuring 7 cm in diameter are 24‐31 yr old. We found no mortality of incubated colonies and calculated the mortality rate to < 7.3 × 10−5 d−1 and the life span to > 24 yr in accordance with age of the largest colonies. The firm gelatinous tissue of high mass density, low nutrient content, and antibacterial property can account for the low growth and mortality rates. The measured daily surplus of photosynthesis minus respiration can support only small increments of the large mass per colony surface in accordance with the low growth rates. Growth and mortality rates of N. zetterstedtii correspond best with general allometric relationships for macroalgae using surface area to carbon biomass as the scaling parameter, but measurements are lower than predictions probably because of extra costs to sustain the substantial colony matrix. N. zetterstedtii is adapted to pristine Lobelia lakes with clear water and chronically low nutrient levels, but it is threatened by strong attenuation of light by brownification or eutrophication.