Colored Dissolved Organic Carbon Research Articles

Dissolved organic carbon as a driver of seasonal and multiyear phytoplankton assembly oscillations in a subtropical monomictic lake

AbstractPhytoplankton assembly dynamics in lakes are highly sensitive to variability in climate drivers and resulting physicochemical changes in lake water columns. As climate change increases the frequency of major precipitation events and droughts, many lakes experience increased inputs of colored dissolved organic carbon (CDOC) and nutrients. How these CDOC‐related changes in resources, transparency, and thermal stability affect phytoplankton assemblages, succession, and resilience is understudied, particularly in subtropical lakes. Here, we used time series, multivariate, and trait‐based functional redundancy analyses to elucidate the roles of phytoplankton in ecosystem resilience and determine potential drivers of assemblage shifts in a subtropical monomictic lake with fluctuating CDOC inputs (Lake Annie, Highlands County, Florida, USA). We found that phytoplankton assemblages and successional patterns differed between two dark‐water states (late 2005–mid‐2007, late 2012–2019) bracketing a clear‐water state (mid‐2007–late 2012), caused by shifting CDOC and nutrient concentrations associated with oscillating groundwater levels. Diatoms (Bacillariophyta), which were dominant during the two dark‐water states, nearly disappeared and were replaced by synurophytes during the clear‐water state. Assemblages had greater interannual consistency in the dark‐water states, while mean functional redundancy decreased in the clear‐water state. Seasonal phytoplankton successional changes were also more pronounced and synchronized with seasonal hydrologic shifts in the dark‐water states. Multiyear assemblage shifts occurred more quickly in clear‐to‐dark than dark‐to‐clear state transitions, suggesting phytoplankton in dark‐water states may be more resistant to state transitions or even contribute to dark‐water state resilience via feedback loops.

A comparison of three colorimetric methods of ferrous and total reactive iron measurement in freshwaters

The measurement of low levels of ferrous and total reactive iron (TRFe) using three common colorimetric methods (bathophenanthroline (BPA), triazine (TPTZ), and ferrozine) in freshwater was evaluated. The BPA method, due to a hexanol extraction step, had unacceptably high limits of detection with small sample volumes. Humic matter also interfered by forming a precipitate. Although ferrozine&s limit of detection was lower, recovery of Fe spikes by TPTZ was much higher in samples with high levels of colored dissolved organic carbon (CDOC) at pH 6 (90 versus 60%) using the standard development time of 10 min because the iron reduction rate is much slower with ferrozine, which requires 1 hr. Recovery of spiked Fe at sample pH 9 was very poor (32 versus 39%). TPTZ with ascorbic acid in samples adjusted to pH 6 is recommended for TRFe analysis. In the absence of a reducing agent and CDOC, TPTZ reagents reduce ferric to ferrous iron (autoreduction), whereas ferrozine has negligible autoreducing properties. CDOC produced significant autoreducing effects using either TPTZ or ferrozine methods, leading to overestimates unless measured immediately upon reagent addition. Hence, immediate measurement with ferrozine is most suitable for ferrous. Ascorbic acid reduces more rapidly than hydroxylamine and does not need to be purified.

Dissolved organic carbon and nutrients as regulators of lake ecosystems: Resurrection of a more integrated paradigm

The primary interpretive paradigm used to study lakes is their trophic status. Oligotrophic lakes have low nutrient loading and low productivity, while eutrophic lakes have high nutrients and high productivity. The strong empirical relationship between nutrient loading and productivity is a valuable tool for teaching, for research, and for management of lakes. In order to incorporate the variety of other known anthropogenic impacts on lakes, however, lake characterization needs to extend beyond the nutrient‐productivity paradigm. For example, acid precipitation, heavy metal and toxic organic contaminants, increases in UV radiation, and global warming are all recognized threats to lake ecosystems. One of the key characteristics of lakes that determines how they respond to disturbances such as these is their concentration of colored dissolved organic carbon (CDOC). Here we argue that a paradigm that includes CDOC (using the absorption coefficient at 320 nm as a proxy) as well as nutrients will be useful in predicting and understanding the response of lake ecosystems to multiple stressors. We propose to resurrect the CDOC axis that was proposed by investigators earlier this century and to extend it by adding some operational definitions to permit placing some of the major lake types on the axes in a way that will help us to better understand the structure, function, and response to disturbance of lake ecosystems that are subject to natural and anthropogenic environmental changes at the local, regional, and global scales. Data from a few diverse lakes and a successional sequence in Glacier Bay, Alaska, are used to illustrate the potential utility of the 2‐axis model in separating lake types.

Carbon cycling in the upper waters of the Sargasso Sea: II. Numerical simulation of apparent and inherent optical properties

A mathematical framework to incorporate spectral apparent and inherent optical properties into a one-dimensional ecological simulation (EcoSim) of the Sargasso Sea is developed. The simulation includes equations for spectral algal particulate absorption, in the form of 4 functional groups of phytoplankton, colored degradational matter (CDM), in the form of 2 classes of colored dissolved organic carbon (CDOC), and downwelling diffuse attenuation coefficients ( K d ’s) at a 5 nm resolution. Particulate absorption responds to changes in phytoplankton species biomass, pigmentation, and photo-adaptation. CDM absorption responds to concentration changes of labile and relict CDOC. K d ’s respond to changes in spectral total absorption, backscattering, and the average cosine of downwelling photons. The spectral bio-optical outputs provide an additional means of validating an ecological simulation. The vertical and seasonal changes in the diffuse attenuation coefficient at wavelengths of 412, 442, 467, 487, 522, and 567 nm compare well with in situ measurements. There appears to be an underestimation of CDM by the EcoSim that is reflected in the simulated absorption and attenuation at 412 nm. The particulate absorption slope between 412 and 487 nm suggests an overestimation of chlorophyll b and photoprotective carotenoid concentrations from Prochlorococcus functional groups. Simulated K d (522) and K d (567) appear to be higher than observations and may result from the exclusion of the inelastic scattering process (i.e. Raman scattering and CDM fluorescence). Results suggest that CDM absorption does not co-vary with particulate absorption. Comparison with a six year time-series of CZCS data suggests that CDM interference of the estimated CZCS chlorophyll a may have been pre-valent in the late spring and the early fall.

Interactions of Photobleaching and Inorganic Nutrients in Determining Bacterial Growth on Colored Dissolved Organic Carbon.

Abstract Bacteria are key organisms in the processing of dissolved organic carbon (DOC) in aquatic ecosystems. Their growth depends on both organic substrates and inorganic nutrients. The importance of allochthonous DOC, usually highly colored, as bacterial substrate can be modified by photobleaching. In this study, we examined how colored DOC (CDOC) photobleaching, and phosphorus (P) and nitrogen (N) availability, affect bacterial growth. Five experiments were conducted, manipulating nutrients (P and N) and sunlight exposure. In almost every case, nutrient additions had a significant, positive effect on bacterial abundance, production, and growth efficiency. Sunlight exposure (CDOC photobleaching) had a significant, positive effect on bacterial abundance and growth efficiency. We also found a significant, positive interaction between these two factors. Thus, bacterial use of CDOC was accelerated under sunlight exposure and enhanced P and N concentrations. In addition, the accumulation of cells in sunlight treatments was dependent on nutrient availability. More photobleached substrate was converted into bacterial cells in P- and N-enriched treatments. These results suggest nutrient availability may affect the biologically-mediated fate (new biomass vs respiration) of CDOC.

Analysis of ocean color components within stratified and well‐mixed waters of the western English Channel

In situ pigment and dissolved organic carbon (DOC) data from two distinct hydrographic regions of the western English Channel are used to explore the possible marine DOC contamination of the past satellite estimates of phytoplankton biomass. To compare with field measurements, the individual spectral contributions of DOC, pigments, and water to the total diffuse attenuation coefficient, Kpar, are summed on a quantum basis within stratified waters near Plymouth, England; and for the spectrally averaged diffuse attenuation coefficient, Kd, on an energy basis within tidally mixed waters near Roscofif, France. In addition, coastal zone color scanner (CZCS) images from 1979 to 1986 were used to compute DOC concentrations for comparison with in situ values. Our analysis suggests that almost 50% of the color signal of satellite‐sensed pigments may be attributed to absorption by marine colored DOC (CDOC) within the English Channel. These results compare favorably to the in situ DOC measurements off Plymouth, but not to off‐Roscoff measurements, suggesting that there may be more CDOC in the stratified waters and more nonabsorbing DOC in the tidally mixed waters.