and nitrogen tracers rather than measurement of nutrient stocks
that are rapidly assimilated and transformed by autotrophs (Furnas
et al., 2005).
Charpy et al. (2012) suggests that relatively low particulate organic
carbon content compared to other lagoons localized at the
same latitude could reflect the impact of pearl oyster aquaculture.
However, this impact does not appear on phytoplankton biomass.
Indeed, as shown by Fournier et al. (2012b), oysters do not feed directly
on phytoplankton, but rather graze heterotrophic plankton.
Fournier et al. (2012b) refined the knowledge on P. margaritifera
diet by demonstrating with the flow through chamber method that
the main factor influencing clearance rates of pearl oysters was the
biovolume of planktonic particles. Thus, the diet of P. margaritifera
was mainly driven by fluctuation of the relative biomass of the
nano- micro- planktonic communities. Both heterotrophic nanoand
micro-plankton represented an important part of the diet of
P. margaritifera depending on their relative biomass in the water
column. The picoplankton communities displayed the lowest clearance
rates but represented however a detectable contribution to
the diet. Whether or not this selective grazing may induce a change
in plankton assemblage in cultivated lagoons compared to uncultivated
ones remain unknown.
Pearl farming could impact lagoons in different ways. First, the
population of oysters hanging on lines may induce changes in the
planktonic communities but this remains unproven to date. Second,
lines hanging above the lagoon floors can modify the flux of material
at the sediment interface. Gaertner-Mazouni et al. (2012) quantified
benthic nutrient fluxes and sedimentation rates for two stations located
under pearl oyster frames, and two control stations away from
the pearl culture facility. They concluded that aquaculture increased
sedimentation rates but probably by modification of local currents
and not by the release of additional organic material. No organic
enrichment in sediments was demonstrated. Conversely, they
showed that maximum values of benthic nitrogen fluxes were recorded
in stations directly under the influence of pearl oyster culture.
These benthic nitrogen fluxes could contribute up to 28% of
the nitrogen demand in the water column. Third, human populations
around farms could directly impact the lagoon. Bouvy et al.
(2012a) concluded from faecal indicator bacteria that there was no
evidence that human sewage had any impact on picoplankton
throughout the atoll. They concluded that Ahe atoll belongs to the
type of unproductive aquatic system, without high external inputs
of inorganic nutrients issuing from human activities, as defined by
Duarte and Agusti (1998).
Three papers in this issue refine knowledge of planktonic communities
of atoll lagoons. First, Bouvy et al. (2012b) investigate
with one survey per atoll the virioplankton and bacterioplankton
in Ahe and Takaroa atolls, in comparison with the surrounding oligotrophic
ocean. The role of virioplankton in lagoons was unknown
while viruses are the numerically dominant biological entities in
the ocean and viral infection is a major structuring process in the
dynamics of marine microbial communities. For instance, viral lysis
of autotrophic and heterotrophic microorganisms influences the
rate of nutrient cycling through microbial food webs. Most virioplankton
in the environment infect bacterioplankton and, in general,
the distributions of viral populations often mirror the
bacterial distributions. However, Bouvy et al. (2012b) suggest that
the distribution patterns of virioplankton are apparently not coupled
in Ahe and Takaroa. Fractions of infected bacterial cells were
all extremely low, among the lowest recorded in both marine and
freshwater systems. Differences between atolls occurred, with a
mean virus-to-bacteria ratio significantly lower in Ahe than in
Takaroa. This is consistent with the hypothesis that this ratio is
likely to increase in environments that favor fast bacterial growth
given the estimated longer residence times in Takaroa compared to
Ahe.