Eutrophic Lagoon Research Articles

Biological treatment of eutrophicated lagoon water with Tetradesmus dimorphus under ambient conditions: a sustainable alternative for lipid production

Biological treatment of eutrophicated lagoon water with Tetradesmus dimorphus under ambient conditions: a sustainable alternative for lipid production

A methodology to assess the phenology patterns of a eutrophic water body, using remote sensing data and principal component analysis

Chlorophyll-a concentrations in coastal and inland waters is acknowledged as an important factor to estimate Water Bodies Quality Status. Remote sensing analysis is a fast, cost-effective, and reliable methodology that can be used to assess the phenological patterns of aquatic ecosystems. The purpose of this study is to assess the phenology patterns of a eutrophic water body and present a novel methodology to fill in the missing values of the daily Surface Reflectance MODIS Aqua dataset. The proposed methodology is to apply Principal Component Analysis to generate the missing values from the chlorophyll-a concentrations and better assess Water Body's Trophic Status. The pilot study is Aitoliko Lagoon, a semi-closed Greek water body with eutrophication. The analysis reveals that sampling stations of swallow waters have higher variance in chlorophyll-a concentrations, favoring the deep waters sampling pixels for evaluating the water body's trophic status. The proposed PCA methodology showed a clear phenological pattern of chlorophyll-a with algae bloom from April and high chlorophyll-a concentrations until August, which is what is expected from a eutrophic lagoon and specifically for Aitoliko lagoon.

Impacts of climate change and best management practices on nitrate loading to a eutrophic coastal lagoon

Anthropogenic climate change and associated increasing nutrient loading to coasts will worsen coastal eutrophication on a global scale. Basin Head is a coastal lagoon located in northeastern Prince Edward Island, Canada, with a federally protected ecosystem. Nitrate-nitrogen (NO3-N) is conveyed from agricultural fields in the watershed to the eutrophic lagoon via intertidal groundwater springs and groundwater-dominated tributaries. A field program focused on four main tributaries that discharge into the lagoon was conducted to measure year-round NO3-N loading. These measurements were used to calibrate a SWAT+ hydrologic model capable of simulating hydrologic and NO3-N loads to the lagoon. Several climate change scenarios incorporating different agricultural best management practices (BMPs) were simulated to better understand potential future NO3-N loading dynamics. Results indicate that all climate change scenarios produced increased annual NO3-N loading to the lagoon when comparing historical (1990–2020) to end of century time periods (2070–2100); however, only one climate scenario (MRI-ESM2-0 SSP5-8.5) resulted in a statistically significant (p-value <0.05) increase. Enlarged buffer strips and delayed tillage BMP simulations produced small (0%–8%) effects on loading, while changing the crop rotation from potato-barley-clover to potato-soybean-barley yielded a small reduction in NO3-N loading between the historical period and the end of the century (26%–33%). Modeling revealed changes in seasonal loading dynamics under climate change where NO3-N loads remained more consistent throughout the year as opposed to current conditions where the dominant load is in the spring. An increase in baseflow contributions to streamflow was also noted under climate change, with the largest change occurring in the winter (e.g., up to a five-fold increase in February). These findings have direct implications for coastal management in groundwater-dominated agricultural watersheds in a changing climate.

Impacts of eutrophication on microbial community structure in sediment, seawater, and phyllosphere of seagrass ecosystems.

Seagrass-associated microbial communities play a crucial role in the growth and health of seagrasses. However, like seagrass meadows, seagrass-associated microbial communities are often affected by eutrophication. It remains unclear how eutrophication influences the composition and function of microbial communities associated with different parts of seagrass. We employed prokaryotic 16S rRNA gene high-throughput sequencing combining microbial community structure analysis and co-occurrence network analysis to investigate variances in microbial community compositions, potential functions and complexities across sediment, seagrass leaves, and seawater within different eutrophic areas of two adjacent seagrass meadows on Hainan Island, China. Our results indicated that microbial diversity on seagrass leaves was significantly lower than in sediment but significantly higher than in seawater. Both sediment and phyllosphere microbial diversity showed no significant difference between the highly eutrophic and less eutrophic sites in each lagoon. However, sediment microbial diversity was higher in the more eutrophic lagoon, while phyllosphere microbial diversity was higher in the less eutrophic lagoon. Heavy eutrophication increased the relative abundance of phyllosphere microorganisms potentially involved in anaerobic metabolic processes, while reducing those responsible for beneficial functions like denitrification. The main factor affecting microbial diversity was organic carbon in seawater and sediment, with high organic carbon levels leading to decreased microbial diversity. The co-occurrence network analysis revealed that heavy eutrophication notably reduced the complexity and internal connections of the phyllosphere microbial community in comparison to the sediment and seawater microbial communities. Furthermore, ternary analysis demonstrated that heavy eutrophication diminished the external connections of the phyllosphere microbial community with the sediment and seawater microbial communities. The pronounced decrease in biodiversity and complexity of the phyllosphere microbial community under eutrophic conditions can lead to greater microbial functional loss, exacerbating seagrass decline. This study emphasizes the significance of phyllosphere microbial communities compared to sediment microbial communities in the conservation and restoration of seagrass meadows under eutrophic conditions.

Intertidal spring dynamics and coastal nutrient loading revealed through geophysics, drone surveys, and in‐situ monitoring

AbstractCoastal eutrophication poses an increasing risk to ecosystem health due to enhanced nutrient loading to the global coastline. Submarine groundwater discharge (SGD) represents a significant pathway for nitrate‐nitrogen (NO3‐N) transport to the coast, but diffusive SGD transport is difficult to monitor directly, given the low flux rates and expansive discharge areas. In contrast, focused SGD from intertidal springs can potentially be sampled and directly gauged, providing unique insight into SGD and associated contaminant transport. Basin Head is a coastal lagoon in Prince Edward Island, Canada that is a federally protected ecosystem. Nitrate‐nitrogen is conveyed from agricultural fields in the contributing watershed to the eutrophic lagoon via intertidal groundwater springs and groundwater‐dominated tributaries. We used several field methods to characterize groundwater discharge, nutrient loading, and in‐channel mixing associated with intertidal springs. The tributaries and intertidal springs were gauged and sampled to estimate a representative summer nitrate load to the lagoon. Our analysis revealed that NO3‐N export to the lagoon through tributaries and springs throughout summer 2023 was on average 401 kg N/month, with the combined spring loading comparable in magnitude to the combined tributary loading. We collected thermal infrared and visual imagery using drone surveys and found spatial overlap between cold‐water plumes from the spring discharge and macroalgae blooms, indicating the local thermal and ecosystem impacts of the focused SGD. We also mapped the electrical resistivity (salinity) distribution in the water column around one large spring with electromagnetic geophysics at different tidal stages to reveal the three‐dimensional spring plume dynamics. Results showed that the fresher spring water floated above the saline lagoon water with the brackish plume oriented in the direction of the tidal current. Collectively, our multi‐pronged field investigations help elucidate the hydrologic, thermal, and nutrient dynamics of intertidal springs and the cascading ecosystem impacts.

Evaluation of Scenedesmus dimorphus under Different Photoperiods with Eutrophicated Lagoon Water

Given the need to improve bioenergy production processes, it is necessary to focus on low-cost culture media and environmental conditions of radiation and temperature. The Scenedesmus dimorphus species was cultured in eutrophicated lagoon water and Bayfolan 0.3% as culture media under four photoperiods with the objective of evaluating the biomass productivity, bioremediation capacity and influence of illumination on the composition and lipid content. It is concluded that the increase of light hours in the culture with eutrophicated lagoon water produces a decrease in the biomass productivity and COD removal percentage. The highest biomass productivity was obtained in photoperiod F1 (10.5:13.5) hours L:O, 0.053 ± 0.0015 g/L day and a removal of 95.6%. Bayfolan 0.3% with F2 (11.5:12.5) and F3 (12.5:11.5) did not show significant differences in the biomass productivity and COD removal. The increase in light hours in the photoperiod induced an increase of 1.01% and 2.84% of saturated fatty acids and 0.8% and 2.14% of monounsaturated fatty acids, as well as a decrease of 3.85% and 2.88% of polyunsaturated fatty acids in eutrophicated lagoon water and Bayfolan 0.3%, respectively.

Impact of agricultural practices on ecosystem health of lagoons: a case study of the Keta Lagoon Complex in Ghana, West Africa

The paper reports on the current ecological health of the Keta Lagoon Complex by analyzing a suite of physical, chemical, and biological environmental conditions prevailing within it. Results are discussed in relation to the main human activity (i.e., agriculture) prevailing in its catchment. The current water quality of the lagoon has declined compared to data collected 20years ago with elevated levels in parameters such as nitrate, phosphate, turbidity, and temperature. Secchi disk depth, salinity, and dissolved oxygen levels in the lagoon have decreased. More than 60% of the total area of the lagoon is predicted to be currently unsuitable to support aquatic life. The Carlson's TSI estimated for the various zones of the lagoon ranged between 72.40 and 80.61 depicting a highly eutrophic lagoon, with about 90% of the total area studied confirmed to be experiencing some degree of eutrophication. The plankton index of biotic integrity also yielded high values of between 3 and 6 in most areas of the lagoon, thereby affirming the poor health of the lagoon. The diversity of phytoplankton and benthic macroinvertebrate species in the lagoon has reduced significantly in the last two decades with about 11 genera of phytoplankton missing in the present study. Benthic macroinvertebrate assemblage richness, evenness, and diversity have decreased from 3.6, 2.0, and 5.8 in 2008 to 1.2, 0.8, and 1.7 respectively, in the present study. Overall, the health of the Keta Lagoon remains poorer and continues to decline with no signs of recovery.

Coastal micro-phytoplankton community changes during the toxigenic Alexandrium minutum blooms in a semi-enclosed tropical coastal lagoon (Malaysia, South China Sea)

Estuarine lagoons often experience eutrophication due to anthropogenic activities in proximity and poor water exchange. Deterioration of water quality in the lagoon has driven the shifts in micro-phytoplankton communities that often form coastal harmful algal blooms (HABs). This study investigated changes in the micro-phytoplankton community in a tropical semi-enclosed lagoon, Geting Lagoon, for two consecutive years during the study periods of 2014/2015 (September 2014–March 2015) and 2015/2016 (August 2015–March 2016). A total of 38 phytoplankton taxa were identified throughout the sampling periods, with 17 taxa of harmful algae identified microscopically. The diatoms Skeletonema costatum and Chaetoceros were predominant in the lagoon. The lagoon was significantly enriched in macronutrients, and high overall nitrogen and phosphorous concentrations likely supported substantial phytoplankton blooms in the lagoon throughout the studied periods. It was observed that with similar temperatures and nutrient availability between the 2014/2015 and 2015/2016 periods, the density of A. minutum in 2014/2015 did not reach as high as in those of the 2015/2016 period, in which higher precipitations, streamflow, and decreased in salinity stratification index had prevented the development of A. minutum bloom. The findings of this study revealed that the coastal micro-phytoplankton community was dependent on the interplay of environmental variables and unique strategic adaptations of the functional types in the assemblages. This study provides valuable insights into the phytoplankton community dynamics in a tropical eutrophic lagoon.

A natural microalgae consortium for the biological nutrient removal in a upflow photobioreactor as tertiary wastewater treatment

A natural microalgae consortium from a eutrophic lagoon was used and evaluated for the tertiary treatment in a upflow photobioreactor. The photobioreactor was continuously fed with the effluent of a UASB high-rate reactor. The photobioreactor average concentration of the inflow pre-treated wastewater in terms of chemical oxygen demand (COD), nitrogen (N), and phosphorus (P) was 125 mg/L, 49 mg/L, and 8 mg/L, respectively. The operation variables of the photobioreactor include a hydraulic retention time (HRT) of 1 day and 2 days of solids retention time (SRT). The photoperiod in the photobioreactor was 24 h at a light illuminance of 5000 lx. The highest nutrient uptake rates in the photobioreactor were 14 mg TN/L·d and 2 mg TP/L·d. Environmental conditions in the photobioreactor favour the cyanobacteria dominance over the microalgae taxonomic groups. In addition, the outcomes established CO2:N ratios of 9:1 to ensure the N removal in wastewater microalgae treatment systems.

Long-Term Water Quality Modeling of a Shallow Eutrophic Lagoon with Limited Forcing Data

Long-Term Water Quality Modeling of a Shallow Eutrophic Lagoon with Limited Forcing Data

Joint analysis of macrofaunal and meiofaunal assemblages improves the assessment of lagoonal environmental heterogeneity

Matching datasets of macrofaunal and meiofaunal assemblages were jointly analyzed for the first time in a coastal lagoon (Cabras Lagoon, Sardinia) in relation to the lagoonal environmental heterogeneity. Within the common faunal list, indicator species/taxa for different environmental conditions were found evenly for both assemblages, with the two faunal components responding distinctly to the putative environmental gradients according to their autecology. The relationship between the number of individuals and species/taxa richness of both assemblages showed an exponential increase at increasing salinity and DO concentrations in water as expected basing on the confinement theory. Along the trophic gradient, the two metrics of both macrofaunal and meiofaunal assemblages showed a similar belt-shaped curve in agreement with theoretical predictions and quantitative field studies, with a peak at intermediate OM values followed by a sharp decrease at OM above 12% at the most confined site. While not a causality measure, our results indicate that these simple benthic-OM relationships applied to both faunal components represent a valuable screening-level indicator for evaluating the likelihood of reduced sediment quality and associated bioeffects in eutrophic lagoons. The present study demonstrates that macrofaunal and meiofaunal assemblages in coastal lagoons, as commonly shown in marine waters, are coherent and distinct entities and that their joint use represents a more comprehensive and robust tool to assess the environmental heterogeneity and quality of these highly variable systems.

Living under threat: Will one of the last Pinna nobilis populations be able to survive?

Abstract The critically endangered fan mussel Pinna nobilis, endemic to the Mediterranean Sea, faces a drastic population decline all over the Mediterranean Sea owing to an infectious disease. Coastal lagoons represent one of this species’ last reservoirs; thus, knowledge of the population conservation status in these ecosystems is essential. An extensive monitoring programme was conducted during 2014, 2017 and 2019 in the Mar Menor lagoon, a highly eutrophicated lagoon from Spain. On average, 32,535 m2 have been surveyed each year to determine population densities, distributions and size structures. The cumulative impacts produced by intensive agriculture activity around the lagoon, together with a catastrophic meteorological phenomenon that occurred in 2019, have triggered a significant decline in fan mussel population densities and in population mean shell sizes over the years. Additionally, some fan mussels were infected by the protozoan Haplosporidium pinnae. Therefore, the current P. nobilis population from the Mar Menor is mainly composed of young individuals in specific areas that are facing an extreme situation. Urgent management and conservation actions focused on restoring the ecosystem and protecting P. nobilis individuals are urgently needed, together with reinforcement projects to increase the species’ stock in one of its last reservoirs.

Dissolved organic carbon accumulation during a bloom of invasive gelatinous zooplankton Mnemiopsis leidyi in the northern Adriatic Sea; case of the anomalous summer in 2017

In the so-called “anomalous” summer in 2017, an increased concentration of DOC was measured in the entire water column at the station SJ107 (eastern part of the Po River delta-Rovinj transect, Graphical abstract) in the northern Adriatic (NAd). Surface DOC ranged between 1.56 and 3.10 mg l−1, being 2.5 times higher than the long-term (1989–2017) average for the same area. Such enlarged concentration of DOC, typical for eutrophic lagoons or lakes and not for open waters, was most probably a consequence of an intensive bloom of Mnemiopsis leidyi, which was recorded with an exceptionally high population density (up to 76 ind. m−3) in the same waters. The number of jelly combs showed a decrease from the east (SJ107) to the west (SJ101) of the transect (Graphical abstract). Abundance in the central part of the transect was also quite low. Maximum of DOC corresponded with the abundance of the M. leidyi smallest specimens (<3 cm). At the western station of the same profile (SJ101), M. leidyi abundance was insignificant, and DOC concentration appeared to be controlled by seasonal changes in primary production and grazing activities. In contrast, at the eastern station (SJ107), changes in the complex food web interaction shaped the DOC quantity and quality. A large fraction of DOC at both stations was found to be reactive, significantly contributing to the organic matter pool represented by surface-active substances (SAS). A large anticyclonic gyre which developed from April to November in 2017 in the area of SJ107, supported calm water column conditions and abundant resources of zooplankton, which could support the residence and accumulation of the invasive ctenophore M. leidyi. For the first time, the presence of a long-lasting gyre (duration of at least 4 months) is documented both with hydrographic measurements and by numerical modelling. Such a situation characterised by the production of highly reactive DOC leads to an extreme eutrophic episode. A dispersion model indicated a high probability of M. leidyi spreading by the end of winter from the Venice Lagoon, its hypothetical winter residing area, to the Istrian coast (Croatia).

Dynamics of primary productivity in relation to submerged vegetation of a shallow, eutrophic lagoon: A field and mesocosm study.

Aquatic ecosystems nowadays are under constant pressure, either from recent or historical events. In most systems with increased nutrient supply, submerged macrophytes got replaced by another stable state, dominated by phytoplankton as main primary producer. Yet, reducing the nutrient supply did not yield the aimed goal of restored habitats for submerged macrophytes in systems worldwide. The question arises, why submerged macrophytes do not re-colonize, and if they are actually competitive. Therefore, primary production assays were conducted in ex-situ bentho-pelagic mesocosms and compared to the actual ecosystem, a turbid brackish lagoon of the southern Baltic Sea. Mesocosm were either manipulated to be colonized by macrophytes, or stayed phytoplankton dominated. Oxygen evolution was monitored over a period of five months in 5 min (mesocosms) to 10 min (ecosystem) intervals. Surface and depth-integrated production was calculated to analyse seasonal and areal resolved production patterns. It was found that macrophyte mesocosms were more stable, when considering only surface O2 production. However, calculating depth-integrated production resulted in net-heterotrophy in both shallow mesocosms approaches and the actual ecosystem. This heterotrophy is likely mediated by sediment respiration and POC accumulation in mesocosms, and a low share of productive to respiring water column in the actual ecosystem. Therefore, it seems unlikely that macrophytes will re-settle, as constant net-heterotrophy may allow for high-nutrient turnover at sediment-water interfaces and within the water column, favouring phytoplankton. These results will assist decision makers in developing more effective restoration measures that can mitigate the negative effects of eutrophication on ecosystem function and services.

Amphipods’ grazing and excretion loop facilitates Chara contraria persistence in a eutrophic lagoon

Charophyte stands represent valuable ecosystems threatened by eutrophication-related phytoplankton and epiphyte blooms. The mechanisms allowing the persistence of charophytes in eutrophic environments are important but poorly studied. We analyzed the benthic primary production and respiration, and inorganic nitrogen (N) fluxes in a Chara contraria stand within a eutrophic estuarine system (Curonian Lagoon) via light and dark intact core incubations. Rates were contrasted with those measured in adjacent unvegetated sediments. Additionally, O2 production was measured in fragments of C. contraria and in the associated community of epiphytes, whereas O2 respiration and ammonium (NH4+) excretion rates were measured in incubation of amphipods (Pontogammarus robustoides) alone. The results from core incubations suggest high primary production and respiration within charophyte stands, resulting in pronounced daily O2 variations in the overlaying water. The incubations of charophyte fragments and associated epiphytes indicate that the latter supports a major fraction of the benthic primary production. The large production of dinitrogen in the dark indicates the occurrence of denitrification, which is suppressed in the light due to primary producers-bacteria competition. On a daily basis, the charophyte stand was a sink for inorganic N via assimilative NH4+ uptake and a dark nitrate sink via denitrification. Interestingly, active NH4+ excretion by the abundant amphipods supported nearly 40 % of the N uptake by the primary producers. We speculate that amphipods continuously feed on epiphytes growing on charophytes, favouring C. contraria persistence and recycling large amounts of N via herbivory and excretion that are immediately re-used by benthic primary producers. This grazing, excretion and assimilation loop may represent an example of macrofauna-macrophyte mutual facilitation and a mechanism easing C. contraria competition under eutrophic conditions.

Seasonal Variation in Biomass and Production of the Macrophytobenthos in two Lagoons in the Southern Baltic Sea

Baltic coastal lagoons are severely threatened by eutrophication. To evaluate the impact of eutrophication on macrophytobenthos, we compared the seasonal development in macrophytobenthic composition, biomass and production, water column parameters (light, nutrients), phytoplankton biomass and production in one mesotrophic and one eutrophic German coastal lagoon. We hypothesized that light availability is the main driver for primary production, and that net primary production is lower at a higher eutrophication level. In the mesotrophic lagoon, macrophytobenthic biomass was much higher with distinct seasonal succession in species composition. Filamentous algae dominated in spring and late summer and probably caused reduced macrophytobenthic biomass and growth during early summer, thus decreasing vegetation stability. Light attenuation was far higher in the eutrophic lagoon, due to high phytoplankton densities, explaining the low macrophytobenthic biomass and species diversity in every season. Areal net primary production was far lower in the eutrophic lagoon. The “paradox of enrichment” hypothesis predicts lower production at higher trophic levels with increased nutrient concentrations. Our results prove for the first time that this hypothesis may be valid already at the primary producer level in coastal lagoons.

Temporal changes of a food web structure driven by different primary producers in a subtropical eutrophic lagoon

Coastal lagoons are often characterized by eutrophic conditions which are known to impair the structure and functioning of both pelagic and benthic compartments. However, the manner in which eutrophication triggers a series of cascade effects in the whole food web in coastal lagoons has received little attention. Using stable isotope (SI) analyses, we investigated the food web structure in the hypertrophic lagoon of Yundang (Xiamen, China) in two periods of the year characterized by the recurrent alternation of Ulva lactuca and phytoplankton blooms in the cool (March) and warm (September) seasons, respectively. Large temporal fluctuations in the dominance of primary producers (i.e. macroalgae vs. phytoplankton) and, thus, in the available food items, were reflected in major changes in the diet and SI signals of several primary consumers, such as the amphipod Grandidierella japonica, the polychaetes Neanthes japonica and Capitella capitata, and omnivorous fishes (i.e. Mugil cephalus, Oreochromis niloticus, and Sardinella zunasi), while these changes were limited in top carnivorous fishes, such as Lateolabrax japonicus. Furthermore, reduced macrozoobenthic abundance available for omnivores in September was found to force omnivores to switch their feeding habits to those of herbivores. The present study provides evidence that the periodical alternation of macroalgal and phytoplankton blooms throughout the year strongly affect the relations among different trophic levels leading to a cascading effect across the whole food web and to major changes in the lagoon's food web structure. Importantly, our study shows that the lagoon's food web structure under persistent eutrophic conditions can still cope with seasonal changes in primary energy source type from macroalgae to microalgae due to the ability of omnivorous fishes to conduit different food sources up to the highest trophic levels. Thus, this study suggests that in such a highly variable eutrophic system, omnivores play a central role in the lagoon's functioning, and help to sustain the biological resources and the ecosystem services provided by the lagoon.

Biodiversity of Marine Microbes

The Special Issue entitled “Biodiversity of Marine Microbes” aimed at highlighting the significance of marine microbes as primary producers, their participation in complex processes and interactions with both the biotic and the abiotic environment, and their important roles in biogeochemical cycles and ecosystem functioning. The issue includes five research papers, covering the diversity and composition of marine microbial communities representing all three domains of life in various marine environments, including coastal eutrophic areas, ice waters, and lagoons. One paper examined the diversity and succession of bacterial and archaeal communities from coastal waters in mesocosm experiments. The combination of classical tools with novel technological advances offers the opportunity to answer fundamental questions and shed light on the complex and diverse life of marine microbes.

Phosphorus Dynamics in a Eutrophic Lagoon: Uptake and Utilization of Nutrient Pulses by Phytoplankton

Coastal ecosystems are subject to human impacts both through adjacent lands and by run-off from the broader catchment area. Nutrient inputs, like phosphorus, were often lowered decades ago, but many aquatic ecosystems have not recovered accordingly. Therefore, restoration and future protection measures need to be founded upon insights on the nutrient demands of phytoplankton in order to limit their growth and negative impacts upon coastal water bodies. This study investigated phosphatase activity, phosphate uptake, and growth of phytoplankton in a model lagoon system at the southern Baltic Sea coast. This lagoon system is perennially dominated by Cyanobium, a clade of picocyanobacteria which lack nitrogen fixation but which are common in brackish waters. Dissolved phosphate and total nutrient ratios from long-term monitoring point to an overall P-limitation of the system. Apparent phosphatase activities confirmed P-starvation. However, experimental phosphate-fertilisations repeated over an annual cycle increased chlorophyll concentration only when either dissolved nitrogen, or N-fixing cyanobacteria were present. In contrast to chlorophyll, particulate matter increased within a week most of the time at the observed stations, even in unfertilised samples. The difference between chlorophyll and particulate responses could be explained by chlorosis after phosphorus fertilisation, as nitrogen was co-limiting. Similar findings have been described for other water bodies. This study indicates that nitrogen is co-limiting in the lagoon system, and that phosphorus input has not yet declined sufficiently.

Variations in Colloidal DOM Composition with Molecular Weight within Individual Water Samples as Characterized by Flow Field-Flow Fractionation and EEM-PARAFAC Analysis.

Fluorescence excitation emission matrices (EEM) and parallel factor (PARAFAC) analysis have been widely used in the characterization of dissolved organic matter (DOM) in the aquatic continuum. However, large sample sets are typically needed for establishing a meaningful EEM-PARAFAC model. Applications of the EEM-PARAFAC technique to individual samples require new approaches. Here, flow field-flow fractionation (FlFFF) combined with offline EEM measurements and PARAFAC analysis was used to elucidate the dynamic changes in DOM composition/optical properties with molecular weight within individual samples. FlFFF-derived size spectra of ultrafiltration-isolated colloidal DOM show that peak-C related fluorophores(Ex/Em= 350/450 nm) are present mostly in the 1-3 kDa size range, while peak-T associated fluorophores (Ex/Em = 275/340 nm) have a bimodal distribution with peaks in both the 1-3 kDa and the >100 kDa size fractions. The integrated EEM spectra from FlFFF size-fractionated subsamples closely resembled the bulk EEM spectra, attesting to the convincing comparability between bulk and FlFFF size-fractionated EEMs. The PARAFAC-derived DOM components are distinctive among individual samples with the predominant components being humic-like in river water, but protein-like in a highly eutrophic lagoon sample. This compelling new approach combining FlFFF and EEM-PARAFAC can be used to decipher the dynamic changes in size spectra and composition of individual DOM samples from sources to sinks or across the redox/hydrological/trophic interfaces.