Hydrolytic enzyme activity, surfactancy, and dissolved organic matter in the digestive lumens of 19 benthic echinoderm and polychaete species were examined, using consistent and quantifiable methods. Enzyme activities were compared with those of extracellular enzymes from ambient sediments. Enzyme activities ranged over five orders of magnitude, with averages decreasing in the order polychaetes > echinoderms > sediment. Highest activities in animals were usually associated with the fluid phase in midgut sections, with posteriorward decreases indicating little export to the external environment. At some phyletic levels, activity correlated inversely with animal size. Hydrolase patterns reflected food type; for example, high 1ipase:protease ratios in carnivores reflected esterified lipids in their diets. High surfactant activity was found in gut sections having high enzyme activity. Deposit feeders had the most intense surfactancy, including evidence for micelles. While enzymes reflected the biochemical nature of the digestible food substrate regardless of feeding mode (e.g., deposit vs. suspension feeder), surfactants reflected dilution of this digestible substrate with mineral grains. Dissolved organic matter levels were high, with amino acids reaching levels > 1 M and lipids commonly 1 g L-r. Among polychaete deposit-feeders, low molecular weight amino acids reflected the composition of the food substrate, but were present at much higher concentrations than could be explained by sediment present in the gut-suggesting longer residence times for fluid than for transiting sediment particles. Deposit feeder digestive fluids are better able to solubilize sedimentary food substrates than are sedimentary extracellular enzymes, owing to either more powerful solubilizing agents or to their deployment in freely diffusing, dissolved form. Gut environments may lead to chemical condensation as well as solubilization reactions.

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Irradiation of particulate organic matter (POM) at light intensities found at the earth's surface should induce reduction in molecular weight, as found for dissolved organic matter, and hence result in transfer to the dissolved phase. We studied Mississippi River suspended sediments to test if photodissolution can induce losses of POM similar to those observed between delivery and burial in coastal sediments. Irradiation experiments in a solar simulator demonstrated dissolution of tens of percent of the POM after several days of exposure to strong sunlight. Neither water type nor iron oxyhydroxide removal had large effect on the reaction extent, but temperature may be a strong controlling parameter. Ultraviolet and visible wavelengths drive this reaction. A hyperbolic response of reaction extent to photon flux allows significant reaction to occur in highly turbid suspensions, despite significant light penetration into the suspensions of only millimeters to centimeters. Our data do not yet allow quantitation of this reaction's contribution to POM loss between the Mississippi River and its depocenter, but they do demonstrate its potential significance under nearshore resuspension regimes. More importantly, these results point to a heretofore ignored role for photodissolution of particulate organic matter at the earth's surface.

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