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[1] We investigated the export of particulate organic matter (POM) to the ocean by two contrasting small, mountainous rivers, the Umpqua and Eel Rivers, by collecting suspended sediment samples over a range of discharges and analyzing them for a variety of constituents, including organic carbon, nitrogen, biomarkers with distinct biochemical sources, and isotopic compositions (d 13 C and Δ 14 C). Concentrations of all measured constituents in both rivers increased as a function of discharge, resulting in their export being dominated by short-lived, wintertime high-discharge events. In the Umpqua River, marked compositional contrasts between low-and high-discharge conditions were consistent with a shift in the provenance of POM from biogenic sources dominated by non-vascular plant sources at low flows to contributions from vascular plant sources of moderate 14 C ages (~300 years before present) dominating at high flows. In contrast, POM from the Eel River, which was highly diluted by mineral sediment at all discharges, had significant contributions from petrogenic sources and displayed lower concentrations of recognizable biomarkers. Both rivers had comparable yields of biogenic POM, which appeared to be moderately degraded and originated primarily from surface soils in erosion prone areas of the watersheds. While tectonic/ geologic differences help explain the contrasts in sediment and petrogenic POM yields between the two watersheds, ecological factors such as vegetation coverage, productivity, and soil carbon are more important in influencing the composition of biogenic POM mobilized from these systems.

The principle of lost opportunity from optimal foraging theory, coupled with recent information about fluxes in the deep sea, allows prediction of feeding behaviours potentially specific to deep-sea deposit feeders. One possible strategy, thus far documented only indirectly, is to ‘ squirrel ’ away rich food from the seasonal or episodic pulses that recently have been shown to fuel meiofaunal growth. Echiurans and sipunculids show morphological and faecal handling patterns consonant with this suggestion. Where it is prevalent, this foraging strategy can have profound effects on stratigraphy. Autocoprophagy is another expected behaviour across a wider taxonomic spectrum, but one that is especially difficult to document. The principle of lost opportunity also predicts highly selective ingestion, not necessarily accomplished by the assessment of individual particles but possibly through pit building in areas where fluids move near-bed material. Under many depositions regimes, small but abundant feeding depressions may be the primary sites where deposition occurs. Conversely, digestive utilization of heterogeneous refractory substrates like humic acids seems as unlikely as an effective municipal waste recycling system that starts with mixed garbage. High gut: body volume ratios in deep-sea deposit feeders, rather than representing an adaptation to use this heterogeneous and refractory end of the food spectrum, instead may allow (through greater residence time of ingested material) greater conversion and absorption of the labile fraction of sediments as it becomes scarcer. Intense natural selection for particle selection ability in fact is one possible reason for the prevalence of meiofauna in the deep sea, and for the diminutive size of macrofaunal taxa there. This selective pressure probably imposes a very restrictive bottleneck on the initial developmental stages of deposit feeders.

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The potential for particle size-dependent bioturbation rates was experimentally tested at 1,240 m in the Santa Catalina Basin (eastern Pacific). Spherical glass bead tracers in five size classes (8-16,17-3 1,32-62,63-l 25, and 126-420 pm) were spread over the sediment surface and tube cored 997 d later. Downcore concentrations of' glass beads were enumerated in each of the five size categories and Page's L-test was used to test the null hypothesis of equal vertical penetration of all size classes of tracer. In all cores the null hypothesis was rejected; finer tracers penetrated deeper into the sediment. In two of the three cores, vertical biodiffusivities were computed from concentration profiles of downcore tracers. These also showed size dependence, with biodiffusivities ranging from 1 cm2 yr-l for the 8-16-pm fraction to 0.1 cm2 yr-' for the 125-420~km size class. These data demonstrate that vertical bioturbation rates are particle size-dependent in Santa Catalina Basin. The likely cause is preferential ingestion and downward transport of fine particles by deposit Nearly all particles that reach the floor of the ocean are displaced several times by animals before they are buried to become part of the sedimentary record. This mixing of sediment or bioturbation has profound effects on a wide range of phenomena. Rates of organic matter decomposition, for example, as well as the dissolution of nearly all sedimentary constituents (e.g. CaCO, and Si02) are markedly influenced by the rate of sediment mixing (Berner 1980). Similarly, the distribution of solid and liquid phase nutrients is mediated by bioturbation, implying that there are likely to be strong feedbacks between the style and rate of mixing and the distribution of infauna. The disl Present address: Applied Ocean Physics and Engineering Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543. Acknowledgments I thank C. Smith and P. Jumars for aid during the initiation of this research and dialogue throughout the entire project. This work could not have been accomplished without the aid and expertise of the master and crew of the RV Atlantis II, especially the Alvin group.