Abstract:
Many benthic organisms show aggregated distribution patterns due to the spatial heterogeneity of
niches or food availability. In particular, high-abundance patches of benthic foraminifera have been reported
that extend from centimetres to metres in diameter in salt marshes or shallow waters. The dimensions of spatial
variations of shelf or deep-sea foraminiferal abundances have not yet been identified. Therefore, we studied
the distribution of Globobulimina turgida dwelling in the 0–3 cm surface sediment at 118m water depth in the
Alsbäck Deep, Gullmar Fjord, Sweden. Standing stock data from 58 randomly replicated samples depicted a
log-normal distribution of G. turgida with weak evidence for an aggregated distribution on a decimetre scale. A
model simulation with different patch sizes, outlines, and impedances yielded no significant correlation with the
observed variability of G. turgida standing stocks. Instead, a perfect match with a random log-normal distribution
of population densities was obtained. The data–model comparison revealed that foraminiferal populations in
the Gullmar Fjord were not moulded by any underlying spatial structure beyond 10 cm diameter. Log-normal
population densities also characterise data from contiguous, gridded, or random sample replicates reported in the
literature. Here, a centimetre-scale heterogeneity was found and interpreted to be a result of asexual reproduction
events and restricted mobility of juveniles. Standing stocks of G. turgida from the Alsbäck Deep temporal data
series from 1994 to 2021 showed two distinct cohorts of samples of either high or low densities. These cohorts are
considered to represent two distinct ecological settings: hypoxic and well-ventilated conditions in the Gullmar
Fjord. Environmental forcing is therefore considered to impact the population structure of benthic foraminifera
rather than their reproduction dynamics.
