Environmental Engineering Reference
In-Depth Information
limit for sulfide is estimated to be about 1-2 µM [29]. TCO
2
was calculated
from measurements of alkalinity and pH determined as described by Astor [4].
Chlorophyll was extracted in methanol and read on a Turner Designs fluorome-
ter using standard methods [12, 19]. Primary production was measured using in
situ incubations of samples enriched with
14
CO
2
as described by Muller-Karger
[25].
4. RESULTS
Nutrient and oxygen data have been collected at the CARIACO time series
site every month since November 1995 from 19 to 20 depths. However, the
above estimates of precision and accuracy are only applicable for data obtained
since 1998 by the USF laboratory of Kent Fanning. We restrict our time series
presentations to data collected since that date. Silicate samples were frozen
prior to cruise 70, so silicate data for the Cariaco prior to September 2001 have
not been used. All data used in this report are available from the Cariaco web
site at
http://imars.usf.edu/cariaco/sdmt.html
or from the authors.
4.1 Vertical Profiles
To demonstrate the distribution of nutrients at the CARIACO station, vertical
profiles are presented in Fig. 1 for cruise 96 from January 2004 (upwelling pe-
riod) for the nutrient species nitrate, nitrite, ammonium, phosphate and silicate,
together with oxygen, hydrogen sulfide, chlorophyll a, primary production, and
beam attenuation (light scattering) which in this system near the interface seems
to reflect bacterial abundance [33]. This date was chosen because samples for
nutrients were taken both at the standard depths used for all monthly cruises,
and at additional depths across the interface (on a separate cruise one week
later). The basic features of the system can clearly be seen. Nitrate, nitrite and
silica values all were extremely low in the upper 15-25 m, while chlorophyll
a and primary production were high. For this date, the chlorophyll maximum
was at 25 m and the maximum in primary productivity was observed at 1 m
(Varela, personal communication). At 35 m, which was the base of the surface
warm layer, nutrient values increased noticeably, with a marked primary nitrite
maximum. The nitrate concentrations reached a maximum at 200 m and a small
secondary nitrite maximum was found immediately above the depth where sul-
fide could first be detected. Ammonium was detectable within the suboxic zone,
but started to increase rapidly only after the onset of sulfide. Throughout this
paper, we empirically define the suboxic zone as the layer where oxygen and
sulfide concentrations were both less than about 2 µM. Based on the oxygen
sensor on the CTD, oxygen concentrations less than 2 µM were present below
depths of about 250 m, and colorimetric analysis showed that sulfide concen-
trations exceeded 2µM below about 300 m resulting in a 50 m suboxic layer on
