Dissertation - HQ

Methods 71
4.2 Methods
4.2.1 Sampling scheme
Thirty-six stations were repeatedly sampled around the atoll of Tetiaroa
(149.55ºW, 17ºS – Figure 4.1, right) from May 10 th to May 27 th 2006,
aboard the N. O. Alis. Tetiaroa was chosen because it is relatively close
to a port, yet quite isolated in regard of the surface currents in the
region. Indeed, trade winds blow from the Southeast, entraining water
in the surface, mixed, layer toward the Northwest and West, through
Ekman transport. Tetiaroa is 55 km straight North of Tahiti, the nearest
land upstream is a small active volcano (Mehetia, 2.3 km 2 ) 190 km away.
Beyond that, it is just open sea for 400 km upstream, to the East and
South-East (Figure 4.1, left). The possibilities for exogenous supply of
larvae therefore seem limited.
An isolated atoll
Figure 4.1 Left: situation of Tetiaroa in the Society archipelago. Right: close up
on Tetiaroa (7 km across) and sampling stations. Stations are sampled in order,
from 1 to 36, in less than 3 days.
To describe the large scale distribution of larvae, the stations were
placed on a large, coarse grid around the atoll. The distance between
the atoll coastline and the farthest stations was 25 km. The smallest
distance between stations was 8 km, which is larger than commonly
observed patch sizes 71,162,163 and should have allowed to sample individual
larval patches independently (Figure 4.1, right). In addition, because
the atoll is approximately 7 km across in all directions, the scale of the
hydrodynamical structures it may induce (e.g. eddies) is also of the
order of the grid size. Hence, at each station, the physical structures
sampled were also probably quite different.
At each station physical and biological data were sampled simultaneously.
A 4 m 2 opening, 800 µm mesh, Multiple Opening-Closing Net
and Environmental Sampling System (MOCNESS) allowed stratified
sampling of the planktonic fauna. Net 0 was lowered at 9-12 m min -1
from surface down to the maximum depth, then nets 1-4 were towed
back up and opened sequentially, at 25 m intervals (Figure 4.2). The
Large scale sampling
Synchronous biological
and physical data