Particle delivery to the benthos of coastal Lake Michigan

Abstract

A 2‐D non‐steady state model was applied to measured profiles of 234Th/238U and 90Y/90Sr disequilibria in a shallow (22 m) water column of coastal Lake Michigan. Downward fluxes of 234Th and 90Y were primarily driven by onshore horizontal advection. Concordance between 234Th and 90Y‐derived mass flux estimates from the water column could only be realistically achieved under a nuclide scavenging scenario dominated by direct sorption on bottom or near‐bottom sediment and vertical convection in the water column—not sinking particles. An estimated vertical 234Th/90Y flux ratio of ∼0.31 in the water column agreed with measured 234Th/90Y activity ratios on collected ejecta from bottom dwelling dreissenid mussels (0.26 ± 0.05) and not with water column particles (3.3 ± 1.3). A similar 238U/90Sr parent nuclide activity ratio of 0.30 ± 0.02 suggests that both 234Th and 90Y are scavenged in toto below the maximum sampling depth (17 m) and near the sediment/water interface. Determining the mechanism by which particles are transported to the bottom is important for understanding not only how benthos are supplied with water column material, but also how particle fluxes should be measured and calculated.

How are coastal benthos fed?

“…Nuclide‐derived POC and PP fluxes in this nearshore study were, e.g., ∼ 15 and ∼ 30 times higher than Eadie et al's () trap‐derived estimates of ∼ 75 mg C m −2 d −1 and ∼ 0.7 mg P m −2 d −1 in offshore waters; however, 234 Th and 90 Y fluxes from the water column were corroborated by measured nuclide inventories on the lakebed (Waples ). Furthermore, the use of ratios in Eq.…”

“…Nevertheless, the nuclide‐derived fluxes from the water column represent a gross flux of what is presented to the lake bottom, and a community of filter feeders at carrying capacity might be expected to use this flux fully. Some proof of this is provided by the concordance we observed between 234 Th and 90 Y flux from the water column and 234 Th and 90 Y efflux from mussels on 10 September (Waples ).…”

“…P fluxes in this study (∼ 21 mg P m −2 d −1 ) could have removed this tributary loading over a relatively small ∼ 12 km 2 area. Likewise, alongshore currents generally flowed southward at our study site (Waples ), but supporting local mussel biomass with up‐shore production only shifts the question to how mussels further north are fed, as Fig. illustrates.…”

“…In shallow coastal waters, however, 1‐D steady‐state conditions are rare. In this study, along the western shore of Lake Michigan, Waples () used a 2‐D non‐steady‐state approach to calculate the vertical water column fluxes of both 234 Th and 90 Y as: where the first term on the RHS of Eq. represents the local growth of A D due to radioactive decay; the second term represents the non‐steady‐state change in A D over time; the third and fourth terms represent advective and turbulent diffusive transport of A D in the cross‐shore (horizontal) direction; and the last term represents the flux of A D reintroduced to the water column through resuspension.…”

“…To resolve this issue, it will be necessary to measure POM flux and other particle‐mediated transport in shallow systems confounded by water movement and boundary layer interactions. Recent studies have shown that naturally occurring radionuclides can be used to quantify particle flux, and, just as importantly, distinguish physical processes that are driving particle transport (Waples ).…”