Environmental Engineering Reference
In-Depth Information
a secondary upwelling tends to occur in July). Previous studies [17, 18, 33]
have demonstrated that there are two maxima in microbial activity, one in the
surface euphotic layer and one bracketing the oxic/anoxic interface.
3. METHODS
Hydrographic data were collected using a SeaBird rosette with a coupled
SBE-19 or SBE-25 Conductivity-Temperature-Depth (CTD) and YSI-23-Y
oxygen sensor. In January 2003, the oxygen sensor was replaced with a Seabird
model SBE-43. Discrete oxygen samples were collected in duplicate using
glass-stoppered bottles and analyzed by Winkler titration ( [32] as modified
by [1]). The analytical precision for oxygen is about 1.5 µM, and the limit of
detection is estimated to be about 5 µM [29].
Water samples for nutrients (ammonium, nitrite, nitrate, phosphate and
silicate) were collected from Niskin bottles into 1-L plastic bottles. These
samples were filtered through a 0.8 µm glass fiber filter into clear 60mL
polycarbonate bottles and frozen within minutes of collection. Beginning in
September 2001 the samples were filtered through a 0.8 µm Nuclepore fil-
ter and a separate sample was collected, filtered, and stored unfrozen for
silicate analysis. All samples were transported within 1 to 4 months to
the University of South Florida, where samples were analyzed following
the recommendations of Gordon [15] for the WOCE WHP project for ni-
trate, nitrite, phosphate and silicate analysis and the standard techniques de-
scribed by Strickland and Parsons [32]. For ammonium analyses, we em-
ployed a method based on modifications (developed by Alpkem (now Astoria-
Pacific International, Inc.) and L. Gordon at Oregon State University allow-
ing the manual method to be used in an automated manner) of published
methods [22] and described at:
http://chemoc.coas.oregonstate.edu:
16080/
∼
lgordon/cfamanual/WHPMANw nh4 web.htm# 6.5. Ammonia
.
Detection limits for the nutrients were determined by calculating the con-
centrations in triplicate standards, averaging the results within each triplicate
group, averaging those standard deviations, and finally doubling the averages
to get the detection limits. Through April 2000 samples were analyzed on an
ALPKEM RFA II. For this RFA equipment the detection limits were: for phos-
phate 0.03 µM, for ammonium 0.07 µM, for nitrite 0.02 µM, for nitrate 0.06
µM, and for silicate 0.04µM. Beginning in May 2000 analyses were performed
on a Technicon Analyzer II. Detection limits for this equipment are: for phos-
phate 0.02µM, for ammonium 0.1µM, for nitrite 0.01µM, for nitrate 0.04µM,
and for silicate 0.4 µM.
Sulfide was measured on triplicate samples preserved with ZnCl
2
using
a modification of the Cline method [8, 29]. Precision of triplicate measure-
ments were about
±
5% for concentrations greater than 1 µM. The detection
