Two excretion trials were conducted in September 2018, which is when
nutrients are especially low in the shallow waters of the SBC due to
stratification and warmer seawater
temperatures.
1
For each trial, live
zooplankton were collected in the morning by conducting five surficial
tows through 3 meters of kelp canopy using an 80 µm mesh plankton net.
The zooplankton from each tow were aggregated into a 500 mL container
and brought up to volume with seawater. A 100 mL aliquot of the
zooplankton aggregated sample was preserved in 95% ethanol for species
identification and abundance analysis using a standard zooplankton
analysis procedure.
14
The preserved
zooplankton samples were analyzed within 48 hours of collection in
order minimize the effects of cell degradation. Zooplankton were
counted for relative abundances and taxonomically identified to the
order level.
In order to take into consideration feeding strategies, zooplankton
were incubated in two types of seawater: GF/F filtered (0.7µm,
Whatman), filtered seawater from the Marine Science Research Building
and 80 µm mesh-filtered, ambient seawater from MOHK. Immediately after
the zooplankton were collected, each experimental group was prepared
by pouring 1 L of the corresponding seawater and 50 mL of the
zooplankton aggregated sample into an incubation bottle. For each type
of seawater, control groups containing no zooplankton were also
prepared. Each experimental and control group contained three
replicates, so a total of 12 bottles were incubated during each trial.
All incubation bottles were maintained at a temperature of 18-20°C,
which is representative of the ambient seawater temperature of MOHK at
the time of collection.
The zooplankton were incubated for two hours. Every hour, an aliquot
of seawater was pulled from each bottle through an 80 µm mesh to
ensure that the zooplankton remained in the bottle. Seawater samples
were filtered again through a GF/F filter and analyzed for nutrient
concentrations. Ammonium
(NH
4
+
)
concentrations were fluorometrically measured with a lower detection
limit of 0.05 µM, using a procedure described by Holmes et.
al.
15
Combined nitrate + nitrite
(NO
2
-
+
NO
3
) was measured
with a lower detection limit of 0.2 µM by the UCSB Marine Science
Institute Analytical Laboratory using a Lachat QuikChem 8500 Series 2
instrument. Urea concentrations were determined with a lower detection
limit of 0.05 µM, using a colorimetric method developed by Goeyens et
al.
16
Chlorophyll A (Chl-A) concentrations
were used as a proxy for phytoplankton abundance and were analyzed in
incubation bottles containing zooplankton using a technique described
by Smith et al.
17
