Environmental Engineering Reference
In-Depth Information
2. MATERIALS AND METHODS
2.1 Study Site and Sampling
All results presented are from the CARIACO time series station (10.50
◦
N,
64.67
◦
W) located in the eastern sub-basin of the Cariaco system. Sampling
was conducted aboard the B/O Hermano Gines, operated by Estacion de In-
vestigaciones Mari nas (EDIMAR), Fundaci on La Salle de Ciencias Naturales
located on Margarita Island, Venezuela. Water samples were collected at 15-
18 depths with a Sea-Bird rosette accommodating 12 teflon-lined, 8-L Niskin
bottles. For hydrographic profiling, the rosette included a Sea-Bird CTD, YSI
oxygen probe, Chelsea Instruments fluorometer for chlorophyll-a estimates and
Sea Tec c-beam transmissometer (660 nm).
Peaks in the transmissometer's beam attenuation have been found to be
reliable proxies for bacterial maxima near the interface in the Cariaco Basin,
and sampling depths were adjusted accordingly to resolve these features as
well as time and manpower permitted. Samples were withdrawn from Niskin
bottles under N
2
atmosphere to prevent oxygenation of samples. All samples
used for biological rate measurements were transferred from Niskin bottles to
HCl-washed 1-L teflon-stoppered glass bottles and sealed without headspace
after overflowing
∼
1 volume. Samples for all biological incubations were
then dispensed from these 1-L bottles under N
2
pressure into acid-washed, 40-
ml septa vials (laminated teflon-butyl rubber septa; Pierce Inc.) or into 40-ml
glass-stoppered bottles and sealed without headspace after overflowing. Further
details are presented in [56].
2.2 Phytoplankton Dynamics
Chlorophyll
a
concentrations were measured in methanol-extracted samples
routinely collected monthly from 1, 7, 15, 25, 35, 55, 75 and 100 m before
dawn. Primary production was measured from these same depths by standard
14
C-bicarbonate protocols in samples [62]. For each depth, 1 dark and 3 clear
polycarbonate bottles were spiked with
3.5 µCi of
14
C-bicarbonate and re-
turned to their collection depth on a buoyed array for 4-h exposures to ambient
light fields, representing
∼
33% of total daily irradiance at this latitude. Af-
ter recovery, samples were filtered through Whatman GF/F filters, rinsed with
0.25 ml of 0.48N HCl, placed in scintillation vials and radioassayed. Data were
corrected for isotopic fractionation (x 1.06) and dark assimilation. Daily pho-
tosynthetic rates were estimated from measured hourly rates, photoperiod and
DIC concentrations [62]. Net primary production was seldom detectable below
75 m, so 100 m integrations captured the entire photic zone. Details appear in
[32].
∼
