Water Profile 1. Take Light, DO, pH, Temp profile every 0.5m Use YSI DO probe, pH
meter, and Li Cor light meter. Take the light profile from the sunny side of the boat.
2. Take Secchi depth Lower Secchi disk slowly until you can never see clear boundaries
between white and black quarters, record this distance to the surface of the water as
lower Secchi disk observation. Then pull the Secchi up until you can always see clear
boundaries between white and black quarters, record this distance to the surface as
the upper Secchi observation. Benthic Net Primary Production 1. Measure light,
temperature, percentDO, DO, and pH at 0.5m intervals at the sampling location. 2. Take
10 clean/undisturbed cores. Try to get a uniform distance between the sediment and top
of tube, so the cores have the same volume of water. Cover in boat with tarp to
exclude light. 3. Collect water from the shore of the boat and measure temp,
percentDO, and DO. Save in bucket. 4. Measure light intensity at 0 (out) and 0.5m
depth where the cores will be incubated. 5. Set up HOBO light recorder on the
incubator. 6. For each tube, take initial temp, percentDO, and DO. Before taking DO
measurement, move the DO probe up and down three times to ensure no DO gradient (but
do not disturb sediment). Add, slowly and without bubbling, 10 to 20mL of water (just
the amount needed) to the core from bucket (number 3) to ensure no air space, and
replace the stopper. Measure the distance from sediment to bottom of stopper to the
nearest 0.5cm (column_depth). 7. Place cores 1, 3, 5, and 7 in dark chambers (opaque
tubes), so there are 4 dark and 6 light treatments. 8. Incubate the cores using the
metal structure at saturation light intensity if possible (300 mol per meter squared
per second at 0.5m depth) for about 3h. 9. Before taking DO measurement, move the DO
probe up and down three times to ensure no DO gradient (but do not disturb sediment),
and then measure percentDO, DO, and temperature in each core. Light controls Once a
month (June, July, August), on a sunny day, incubate 10 cores for 3h with different
light intensities to determine primary productivity under different light intensities
and different temperatures. It would be best to do this the day after routine sampling
(i.e., when retrieving the benthic sampler) so that the results can be compared to
those from the routine sampling. Different light levels are obtained using white mesh
bags around the core tubes. Core 1 and 6, light Core 2 and 7, 2x Core 3 and 8, 4x Core
4 and 9, 8x Core 5 and 10, dark IMPORTANT: After the incubations, measure light
intensity inside a core tube covered for the different treatments. This is done by
removing the light meter from the metal holder and placing it facing up in a core
using zip ties and a blue stopper at the bottom. Then place treatment bags over the
top and measure light when holding the core at the level they reach in the incubator;
use the marking on the light meter cord to make sure this is standardized for all
measurements. This should be done 8 times total (each bag plus twice without bags).
Light saturation Once a month in the summer of 2013, we conducted sediment core
incubations with varying amounts of shade cloth applied to the cores. Sediment cores
received 0, 2, 4, 8, or 15 layers of shade cloth, with two cores in each treatment.
All cores were then incubated in the lake over the same 3hr period at a depth of 0.5m.
Sediment Dry Weight and Weight on Combustion 1. Remove 0.75cm of sediment from a core
into a plastic deli container. This should be done on a fresh core. This is the same
sample that is used for chl analysis. 2. Subsample 5 to 10mL sediment solution and
place in a pre-weighed tin tray in oven at 60C for at least 12 hours. When dry, weigh
for dry weight. In 2014, the method for sampling benthic chlorophyll changed. Sediment
Dry Weight measurements were taken from these samples as well. Below is the pertinent
section from the methods protocols. Processing after the collection of the sample was
not changed. Take sediment samples from the 5 cores collected for sediment
characteristics. Take 4 syringes of sediment with 10mL syringe (15.3 mm diameter).
Take 4-5cm of sediment. Then, remove bottom 2cm and place top 2cm in the film
canister. 3. Combust at 550C for 4.5 hours. Weigh tray. 4. If not analyzing combusted
samples immediately, place in drying oven before weighing.
Sonde 30-min measurements of water temperature, conductivity, turbidity, dissolved
oxygen, and phycocyanin from a semi-permanent Hydrolab DS5X sonde at the center of
Lake Myvatn (St. 33). For 2012 and 2013 turbidity was measured only in terms of NTU,
while we added volts in 2014 because this gives better resolution at very low
turbidity. Note: The dissolved oxygen sensor for the sonde malfunctioned in 2014. The
data file shows the sonde readout for DO, but this should not be used. When the sonde
was serviced following the summer of 2014, it was discovered that the turbidity sensor
was likely calibrated incorrectly from initial deployment. Furthermore, Hydrolab has
informed us that the malfunction of one sensor may impact the performance of other
sensors. The sonde was fixed before the start of field season in 2015.
Water Chemistry Nutrient analysis performed on water samples collected during
weekly routine sampling at the center of Lake Myvatn (St 33). Nutrients analyzed
include total phosphorus (2012-present), total nitrogen (2012-present), nitrate and
nitrite (2014-present), ammonium (2014-present), orthophosphate - soluble reactive
phosphorus (2014-present), silica (2012-present), and dissolved reactive silicon
(2012-2013).
