Tracing the Flow of Organic Matter from the Musekgon River Estuary System to Nearshore Lake Michigan: a Stable Isotope Analysis

Abstract

The fate of inputs of organic matter from individual watersheds to the Great Lakes is poorly known. The goal of this study was to track the delivery of organic matter from the Muskegon River Estuary System (MRES) into nearshore Lake Michigan through a stable isotope analysis of water, sediment, and sources of primary production. The MRES is comprised of the lowest 90 km of the Muskegon River watershed from Croton Dam and includes Muskegon Lake, a drowned river mouth lake. Nutrients, total suspended material (TSM) and particular organic matter (POM) were measured from water samples collected monthly from May through October at five stations from Croton Dam to nearshore Lake Michigan, and carbon and nitrogen stable isotope signatures were analyzed. Sediments were collected at several sites in Muskegon Lake and nearshore Lake Michigan in May and September, and their grain size compositions and isotopic signatures were characterized. Values of TSM were significantly higher at the mouth of the Muskegon River (average 9.14 ± 1.67 mg/L) than in nearshore Lake Michigan (average 1.40 ± 0.17 mg/L), suggesting that much of the suspended material from Muskegon River was retained within Muskegon Lake. Isotopic signatures of POM collected in Muskegon Lake were depleted in δ13C (-30.8‰) relative to the isotope signatures of POM from Lake Michigan (-26.2 ‰) or the mouth of the Muskegon River (-28.1 ‰), suggesting an additional source of depleted carbon was present in Muskegon Lake, likely biogenic methane. Sediments in Muskegon Lake were characterized by fine grains (< 63μm) with depleted δ13C signatures (-28.9 ‰) compared to Lake Michigan sediments which were characterized by larger grains (>210 μm) with enriched δ13C signatures (-24.1 ‰). Sediment δ15N signatures increased on a strong east-to-west gradient within Muskegon Lake itself, indicating significant microbial processing of nutrients occurring within the lake. Additionally, the west end of Muskegon Lake was characterized by sediment with higher levels of organic carbon and lower C/N ratios than measured either in Lake Michigan or Muskegon River, indicating an area of extensive aquatic production. The extent of nutrient uptake occurring in Muskegon Lake may have completely altered the chemical and isotopic characterization of organic matter flowing into the lake from Muskegon River. As such, there was no traceable impact of the carbon and nitrogen content of organic matter from Muskegon River to nearshore Lake Michigan.