Several time course storage experiments with preserved seawater samples were conducted to study the loss of bacterial cells as a function of storage time The number of bacteria decreased by 24 to 50'% within 7 to 29 d i n samples preserved with 2.5";* glutaraldehyde [final conc.). A comparison between epifluorescence and electron microscope counts showed that the decrease was not due to filtration artefacts. Only 0.4 to 0.6% of the bacterial cells were found to be attached to the walls of the sample containers after 1 yr of storage. There was no positive correlation between the frequency of virus-infected cells at the start of the storage experiments and the loss of bacteria as a function of storage time. Numbers of bacteria declined by only 5 % the first 9 d in samples preserved in glutaraldehyde and stored at -20°C. By adding phenolmethylsulfonylfluoride (PMSF), a protease inhibitor, prior to the addition of glutaraldehyde, the loss of bacterial cells only 17 to 18' ' over a 30 to 35 d period. Our study shows that protease activity may be a inajor cause of bacterial loss in glutaraldehyde preserved samples.

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Experiments were conducted to study the effect of chloroform addition to sediment traps in the laboratory as well as in the field. Chloroform is an organic solvent which gave rise to extensive leaching of pigments and probably also other soluble organic compounds when fresh organic matter entered sediment traps. Droplets of chloroform suppressed microbial degradation of organic matter in the traps for a few days, but did not stop it. The presence of chloroform resulted in rapid anaesthesia of 'swimmers' such as cladocerans and copepods and their subsequent death. Thus, chloroform led to accumulation and disintegration rather than exclusion or preservation of 'swimmers' The field and laboratory experiments suggested that loss of organic matter in unpoisoned traps was due more to 'swimmers' which entered, grazed and left the trap than to microbial degradation per se. Chloroform does not add significantly to the accuracy of vertical flux data. On the contrary, it gives rise to significant and undesirable side-effects, e.g. over-and underestimation of POC and chlorophyll a sedimentation rates, respectively.