292
Assay of Some Common Hydrobiological Techniques
LOM
follows: T = - - - - - - days, where 0.375 is the transfer coefficient of
(TPM x 0.375)
oxygen to carbon units at RQ = 1. In eutrophic to hypereutrophic or polluted
basins, the stock of oxygen in the experimental bottles may be insufficient for
oxidation of the LOM stock present in the sample In this case, the samples
are diluted 1: 2 or 1 : 4 with bidistilled water before bubbling.
Whole-community respiration rates in shallow productive basins may be
quite reliably evaluated measuring of the decrease of dissolved oxygen
contents in water samples taken twice at fixed stations from two to three
depths on the vertical profile, once in complete darkness and the second
time 3-6h later. During this time period, the oxygen saturation usually turns
its level from oversaturation down to undersaturation. This avoids the need for
correction of the oxygen exchange between water and atmosphere between
the samplings. The average difference in oxygen thus obtained in a sampled
series, mg 0 21- 1 , calculated 1 h- 1 and multiplied per 24 h, gives a good approximation of the rate of whole-community respiration (TCM). Another way
of estimation is measuring TPM in the water column together with estimating
the bottom oxygen uptake rate (TBM), employing black in situ enclosures or
cores (see Sect. 2.5.2.). Whole-community respiration (TCM) is then calculated
as follows: TCM = (TPM x lOR) + TBM mg02l-1day-l, if TBM is the bottom
10H
respiration, mg O 2 dm- 2 day-l and H the depth, m (Sorokin et al. 1996a).
6.4 Determination of Free H 2S and Labile Sulfides
in Bottom Sediments
The contents of free H 2S in bottom sediments is a very important parameter,
which may predict the dangerous phenomenon of the appearance of H 2S in
the near-bottom water layer. This can happen during critical periods of the
midsummer hypolimnetic oxygen minimum or of winter anoxia in coastal
marine basins and eutrophic lakes.
The samples are taken with the aid of a spoon into a wide-mouth 100-200ml plastic bottle from the upper 0-3-cm layer of sediment, fetched by a grab
or corer sampler. The bottles are filled with sediment without air bubbles,
tightly closed with a cap, and brought nonfixed into the laboratory. For up to
2 days, they may be preserved in the refrigerator before being analyzed. A subsample of 15-20cm 3 is taken with a moistened calibrated glass tube and placed
into the vessel of the apparatus for H 2S distillation (see Fig. 5.9) containing
10ml deoxygenated 1 % phosphate buffer solution with pH adjusted to ~6.
The absorber is filled with 8-10ml 1: 1 mixture of 1 % Zn acetate and 0.2N
NaOH solutions. The mixture is shaken before use. The free H 2S is distilled
for 20min by passing neutral gas through the apparatus as described above
Assay of Some Common Hydrobiological Techniques
LOM
follows: T = - - - - - - days, where 0.375 is the transfer coefficient of
(TPM x 0.375)
oxygen to carbon units at RQ = 1. In eutrophic to hypereutrophic or polluted
basins, the stock of oxygen in the experimental bottles may be insufficient for
oxidation of the LOM stock present in the sample In this case, the samples
are diluted 1: 2 or 1 : 4 with bidistilled water before bubbling.
Whole-community respiration rates in shallow productive basins may be
quite reliably evaluated measuring of the decrease of dissolved oxygen
contents in water samples taken twice at fixed stations from two to three
depths on the vertical profile, once in complete darkness and the second
time 3-6h later. During this time period, the oxygen saturation usually turns
its level from oversaturation down to undersaturation. This avoids the need for
correction of the oxygen exchange between water and atmosphere between
the samplings. The average difference in oxygen thus obtained in a sampled
series, mg 0 21- 1 , calculated 1 h- 1 and multiplied per 24 h, gives a good approximation of the rate of whole-community respiration (TCM). Another way
of estimation is measuring TPM in the water column together with estimating
the bottom oxygen uptake rate (TBM), employing black in situ enclosures or
cores (see Sect. 2.5.2.). Whole-community respiration (TCM) is then calculated
as follows: TCM = (TPM x lOR) + TBM mg02l-1day-l, if TBM is the bottom
10H
respiration, mg O 2 dm- 2 day-l and H the depth, m (Sorokin et al. 1996a).
6.4 Determination of Free H 2S and Labile Sulfides
in Bottom Sediments
The contents of free H 2S in bottom sediments is a very important parameter,
which may predict the dangerous phenomenon of the appearance of H 2S in
the near-bottom water layer. This can happen during critical periods of the
midsummer hypolimnetic oxygen minimum or of winter anoxia in coastal
marine basins and eutrophic lakes.
The samples are taken with the aid of a spoon into a wide-mouth 100-200ml plastic bottle from the upper 0-3-cm layer of sediment, fetched by a grab
or corer sampler. The bottles are filled with sediment without air bubbles,
tightly closed with a cap, and brought nonfixed into the laboratory. For up to
2 days, they may be preserved in the refrigerator before being analyzed. A subsample of 15-20cm 3 is taken with a moistened calibrated glass tube and placed
into the vessel of the apparatus for H 2S distillation (see Fig. 5.9) containing
10ml deoxygenated 1 % phosphate buffer solution with pH adjusted to ~6.
The absorber is filled with 8-10ml 1: 1 mixture of 1 % Zn acetate and 0.2N
NaOH solutions. The mixture is shaken before use. The free H 2S is distilled
for 20min by passing neutral gas through the apparatus as described above