On the temporal variation of DOM fluorescence on the southwest Florida continental shelf

Abstract

Dissolved organic matter (DOM) plays a crucial role in the nutrient and carbon cycling in the coastal-shelf-ocean boundary. It is a major reservoir of reduced carbon and carries important information on how sea and landscape have been modified. Due to the complexity of the biogeochemical processes in this boundary, its distribution is not well understood. This study evaluated the Excitation Emission Matrix (EEM) fluorescence method combined with Parallel Factorial Analysis (PARAFAC) modeling to resolve DOM fluorescence components, and their distribution from near-shore to off-shore on the southwest (SW) Florida Shelf. The PARAFAC model derived identified five unique DOM components from 272 Excitation and Emission Matrix (EEM) samples measured across the shelf on May and October 2008, and January and April 2009 cruises. These five different fluorophores consisted of two humic-like components that were attributed to natural (component1: HLC1) and anthropogenic (component 2: HLC2) sources; an unidentified component (component 3); two autochthonous protein components, identified as tryptophan-like (component 4:TLC4); and tyrosine-like (component 5:TLC5) fluorophores. The observed temporal variations of terrestrial and anthropogenic humic-like DOM fluorescent materials were not necessarily dependent on the timing of river runoffs and rainfall frequency on the shelf, but also on other factors (i.e., suspension of sediments in shallow water induced by wind-driven mixing, dilution, hypersalinity conditions). The areal distribution of the resolved terrestrial DOM fluorescence on the mid-shelf indicates evidence of offshore transport of land-derived materials. The evident terrestrial humic-like inputs into the shelf correspond to the increase in the protein-like fluorescence, which was attributed to autochthonous biological degradation. Results from this study demonstrated advances in several aspects of monitoring the temporal distribution of DOM on the continental shelf of Florida using EEM and PARAFAC methods. The methods employed were able to: (1) resolve sources of the individual DOM fluorescence components of the bulk DOM; (2) generate spatial and temporal distribution maps of each of the components on the shelf; and (3) provide insights into biological, chemical, and physical processes that control the DOM variability of each of the resolved DOM components.