Changes in CDOM fluorescence from allochthonous and autochthonous sources during tidal mixing and bacterial degradation in two coastal estuaries

Abstract

Chromophoric dissolved organic matter (CDOM) was collected and concentrated using 1 kDa cutoff tangential flow filtration (TFF) from marine (∼33 salinity), mid-estuarine (∼15 salinity), and freshwater (<1 salinity) portions of the Chesapeake and San Francisco Bays. Natural bacterioplankton were also collected during the same transects on 0.22-μm pore size filters. TFF permeates from freshwater, mid-estuarine and marine stations were used to create a series of salinity samples ranging from 0 to 33 by increments of 3. Freshwater CDOM was added in the same proportion to each salinity sample to determine changes in spectral signals during simulated estuarine mixing. A series of incubations was conducted in which concentrated CDOM was added to TFF permeates (<1 kDa, low fluorescence) in a nine-membered matrix such that each station's CDOM was added to each station's TFF permeate. Each incubation was then inoculated with a filter from its respective collection location. Subsamples from bacterial incubations were collected at various times and analyzed by high resolution three-dimensional fluorescence excitation–emission spectroscopy (EEMs) to determine if changes in ionic strength encountered during estuarine mixing affect the bioavailability and optical properties of CDOM. Five EEMs peaks were identified for each mixing experiment and microbial subsample; Ex max: 330–350 nm/ Em max: 420–480 nm, Ex max: 250–260 nm/ Em max: 380–480 nm, Ex max: 310–320 nm/ Em max: 380–420 nm, Ex max: 270–280 nm/ Em max: 300–320 nm, and Ex max: 270–280 nm/ Em max: 320–350 nm. These peak ratios were monitored over the time course of the experiment. Changes in several spectral properties during the simulated estuarine mixing were observed indicating CDOM conformational changes as it moves through the estuary. We hypothesized these changes may impact the biodegradability of CDOM as it moves from upland sources to the coastal ocean. Changes in DOC concentration during incubation indicated that allochthonous CDOM was a more utilizable substrate for estuarine and marine bacteria. There were also differences in peak ratios observed during incubation with allochthonous and autochthonous CDOM. There were Em max peak shifts dependent on the source of CDOM and bacteria, with more red shifting (toward higher wavelengths) in upper reaches of the estuary and more blue-shifting at the oceanic end-member. We conclude that bacterial degradation of specific components of autochthonous and allochthonous CDOM may impact the spectral characteristics observed throughout an estuary and that CDOM optical properties are partially a function of the CDOM's origin and mixing history.