Quantification of particulate organic matter sources in an eutrophic river estuary by stable carbon and nitrogen isotopes and photopigments (Neuse River Estuary, North Carolina, United States)

In estuarine ecosystems, the deposition of freshly-produced organic matter supplies nutrition for benthic macroinvertebrates and can have a profound effect on overlying water quality once microbially respired. However, some organic matter is unreactive relative to estuarine flushing times. Variations in the sources, abundances, and reactivities of particulate organic matter (POM) were examined during seasonal and episodic (i.e., hurricane) hydrological changes to the eutrophic Neuse River Estuary, North Carolina (USA). I assessed the relative importance of hydrology versus algal production to the sources of POM by analyzing reservoirs having variable lifetimes (seston, settling organics, and sediments) by bulk organic matter tracers (elemental ratios, stable carbon (δ13C) and nitrogen isotopes (δ 15N)) and compound-specific biomarkers (photosynthetic chloropigments and carotenoids). Four primary sources of organic matter were identified (terrestrial, and riverine, estuarine, and sound-derived phytoplankton) by the non-conservative distributions of tracers. Hurricane flooding transported 65% of the annual load which was comprised of forested soils, riverine and estuarine sediments. During normal flows, mid estuarine maxima in δ15N seston (>10‰) indicated a supply of algal POM, not an 15N enriched waste (i.e., wastewater). Deposition of POM depended on phytoplankton community structure. Fucoxanthin (diatoms) and zeaxanthin (cyanobacteria) dominated water inventories, but diatoms were preferentially deposited, ostensibly because of high settling rates and zooplankton repackaging of diatoms. Deposition of algal organic matter was indicated by the strong coupling of chlorophyll a in the water column and surficial sediments. After hurricane scouring reduced surficial sediment chlorophyll a inventories, deposition of chlorophyll a exceeded loss processes in the mid-estuary region. This region is an organic-rich (>0% org C) depositional center, where recent fish mortalities events have been documented (1996–2001). Sources of depocenter POM were the seasonal deposition of 15N enriched phytoplankton and episodic hurricane deposition of a 13C depleted allochthonous organics. Phytoplankton grown under different hydrographic conditions have distinct C and N isotopic signatures which may be used as biomarkers identify organic matter sources contributing to the estuarine food web. Furthermore, this study serves as a current baseline of organic matter processing in an eutrophic estuary in the event of future changes to estuarine trophic status.