Values Of Particulate Organic Matter Research Articles

Source apportionment of suspended particulate organic matter in a shallow eutrophic lake of Southwest China using MixSIAR model

Eutrophication has become prominent in many lakes of the world, resulting in a continuous rise of suspended particulate organic matter (POM) that can be of vital consequence for water quality and carbon cycling. While the accumulation and decomposition of POM can enhance nutrient cycling at a rate depending on the property of lake-water organic carbon, it is important to identify the source of suspended POM for evaluating eutrophication and carbon burial. Here we analyzed the distribution of stable isotopic ratios (δ13C and δ15N) and the carbon/nitrogen ratio (C/N) in suspended POM in a shallow eutrophic lake of subtropical China, for apportioning the POM sources using Bayesian mixing model. Specifically, we collected seasonal samples of lake-water POM (>0.7 μm) from 20 sites in Yilong Lake from September 2020 to June 2022, as well as six types of modern samples covering the main carbon sources in the catchment. The spatial surveys showed that in the dry year (low water level) the δ13C of POM was more enriched in the western basin than those in the central and eastern basins, while the seasonal surveys showed the most enriched δ13C of POM in September. However, this spatio-temporal heterogeneity disappeared in the wet year (high water level). It was also found that the δ13C, δ15N and C/N values of POM varied significantly with those of phytoplankton across sites and seasons. Application of the MixSIAR model further showed that phytoplankton was the predominant source of POM in Yilong Lake (79.8 ± 13.4 %), but with large annual difference, 68.9 ± 10.3 % in the dry year and 90.7 ± 3.7 % in the wet year. In comparison, the contribution of the other five endmembers was much more moderate. In particular, the proportion of allochthonous organic carbon in POM was relatively low, with terrestrial plants and soils contributing 6.1 ± 4.8 % and 4.3 ± 2.5 % of POM, respectively. Our quantitative evidence for the dominance of the autochthonous source (i.e., phytoplankton) in lake-water POM over time and space suggested a determining role of eutrophication in the composition of lake organic carbon. Therefore, the coupling of algal blooms with organic carbon cycling in inland waters can be enhanced with continuous catchment development and regional warming.

Coastal warming heightens direct impacts of seawater temperature on nutrients near aquaculture farms in Korea

The potential roles of temperature and phytoplankton in nutrient cycling throughout the water column were investigated nearby aquaculture farms. Using the convergent cross mapping (CCM), we examined the relative strength of phytoplankton and temperature effects on nutrients. High δ15N values of particulate organic matter in the inner bay were detected compared to those in the outer bay. δ15N values >5 ‰ throughout the bay indicate that nitrogen influxes from the aquaculture farms are the critical nitrogen source in the study region. Our CCM models revealed that temperature positively and strongly affected the potential regeneration of nutrients, associated with PO43− while phytoplankton utilized nutrients as soon as available. The temperature–driven nutrient regeneration was higher in the bottom layer than that in the surface layer, indicating that temperature was a more important controlling factor in nutrient fluxes from the surface sediments.

Investigating food web structure and system function of an artificial reef ecosystem based on carbon and nitrogen stable isotope analysis: implications for reef management

IntroductionFood web is an important basis for identifying trophodynamic processes, and evaluating the structural and functional characteristics of ecosystems. The trophodynamics and system function of artificial reef (AR) ecosystems have rarely been examined.MethodsStable isotope analysis was used to investigate the food web structure and functions of an artificial reef (AR) ecosystem in this study.Results and DiscussionThe δ13C and δ15N values of particulate organic matter (POM) in AR showed noticeable seasonal changes, and the δ13C value of POM in autumn was significantly higher than that in other seasons (p<0.05). There were no significant seasonal variations in the δ13C values of solid organic matter (SOM), and no significant difference between SOM and POM was observed except in autumn. Moreover, macroalgae did not significantly affect the δ13C values of SOM. Phytoplankton may be the primary nutrient source in the AR ecosystem. The δ13C values of most crustaceans in the AR were approximately between (-17.03 ± 0.22) ‰ – (-17.74 ± 0.07) ‰, higher than those of most fish, indicating that they may have different basal nutrient sources. The trophic level (TL) of invertebrates was between 2.00 and 3.09, and that of fish was between 2.98 and 3.66. The distribution of δ13C and TLs of crustaceans and fish showed that, except for crustaceans, bivalve shellfish and zooplankton might also be important food sources for fish in the AR ecosystem.ConclusionThe δ13C and δ15N values of most species in the AR showed good continuity, indicating that they may be mainly produced from the AR ecosystem. Migratory species such as Lateolabrax japonicus and Sepiella maindroni showed higher δ13C values, indicating that they may have migrated from other sea areas. To maintain the stability of the ecosystem structure and function of the AR ecosystem, fishing activities should be carried out following the maximum sustainable yield theory. Future research needs to identify the nutritional relationship between AR and its adjacent sea areas, to depict the food web structure of the AR with higher accuracy.

Impacts of shrubs on soil quality in the native Monte rangelands of Southwestern Buenos Aires, Argentina

AbstractShrub cover in semiarid rangelands may induce changes in soil resources and ecosystem functioning. However, it is unknown the real influence that shrub vegetation has on soil quality in rangelands used for livestock purposes. We evaluated the shrub cover effect on 12 chemical and biochemical parameters of soil quality. In a semiarid Monte rangeland of Argentina, 6 paddocks were selected and 10 m transects were placed in a patch with (Sh) and without shrubs (WSh). Then, sites with grasses (Sh‐G and WSh‐G), bare ground‐litter (Sh‐BL and WSh‐BL), and under shrub cover (Sh‐S) were selected. In spring 2017 and 2018, a composite soil sample (0–10 cm in depth) was taken at each site (n = 6). Sh‐G and Sh‐S sites presented high values of soil organic matter, soil organic nitrogen, particulate organic matter (POM), and cellulase activity (CA); WSh‐BL and Sh‐BL sites were associated with the lowest contents of these variables. For the rest of the soil quality parameters, the soil sampling sites showed similar values. These results show that woody presence should not be directly linked to soil quality loss. Although we did not detect a shrub effect in all parameters studied, in the context of appropriate grazing management, the presence of plant species of different functional groups has a positive effect on organic matter and N content of soil close to them. Moreover, in these sites, high POM values represent an important reservoir of potentially available nutrients, and promote CA necessary for fresh litter decomposition improving the soil quality of semiarid rangelands.

A gulf-wide synoptic isoscape of zooplankton isotope ratios reveals the importance of nitrogen fixation in supporting secondary production in the central Gulf of Mexico

A synoptic gulf-wide isoscape of carbon (δ13C) and nitrogen (δ15N) in the Gulf of Mexico based on mesozooplankton (335-1000 um) was used as a proxy for the isotopic baseline and for calculating regional contributions of dissolved inorganic nitrogen sources. Mesozooplankton were sampled at 0-200 m (depth permitting) during the XIXIMI-06 and GOMECC-3 cruises held during the summer of 2017. A striking latitudinal gradient was found in δ15N values of zooplankton, with the highest values (10.4 ± 1.2‰) found over the northern shelf, and lowest values in the central, oligotrophic gulf (1.9 ± 0.5‰). To estimate the fractional contribution of potential nitrogen sources, the gulf was divided into six regions based on the spatial distribution of surface Chl-a, SST from remote sensing products and likely region-specific source contributions. A literature survey of (δ13C and δ15N values of particulate organic matter was used to characterize region-specific endpoint isotope ratios for use in a Bayesian isotope mixing model. Regional differences in δ15N values and the results of mixing models indicated nitrogen fixation is most likely an important source (45-74% contribution) of new nitrogen in the oceanic regions of the Gulf, the Loop Current and the Yucatan Shelf. In the oligotrophic gulf, the potential input of relatively light nitrate that reflects remineralization of surface layer POM or the excretion of light nitrogen by heterotrophs was insufficient to explain the low δ15N values found in the central Gulf, although it could account for about 40% of the N supporting secondary production. The high nitrogen isotope ratios found in the northern shelf were attributed to denitrified N (60%) and the inflow of heavy nitrogen from the Mississippi-Atchafalaya river system. Our results support the potential importance of fixed nitrogen in the deep waters of the Gulf of Mexico during the summer, characteristic for its highly stratified surface waters.

Investigating terrestrial and oceanic environmental conditions to identify possible factors influencing seaweed bed distribution in Tsushima Islands, Japan

A decrease in seaweed beds has been a serious problem in the coastal areas of the Tsushima Islands, and other areas in Japan and worldwide. In the Tsushima Islands, certain coastal areas in the northeastern (NE) area contain seaweed beds, while their depletion is of serious concern in other areas. The influence of terrestrial and oceanic waters, and their relationship with seaweed bed distribution in the Tsushima Islands, Japan was examined to understand the marine environmental conditions. We conducted comprehensive investigations based on water sampling and their analyses in this study. Water qualities including total iron, dissolved iron, nitrate nitrogen, nitrite nitrogen and phosphate-phosphorus, and carbon stable isotope ratios of particulate organic matter (POM) in seawaters and river waters in the NE and southwestern (SW) areas of the Tsushima Islands were seasonally monitored. The seawater quality results showed no distinct differences between the NE and SW areas. This observation indicates that the materials, including iron supplied from terrestrial areas, were not the primary factors characterizing the seaweed bed distribution, despite the seaweed bed distribution in the NE area being larger than that in the SW area. Meanwhile, the seasonal variations in the δ13C values of POM in the NE area were more evident than those in the SW area. This implies the significant contribution of oceanic POM in the NE area, possibly due to the existence of island-induced current and cyclonic eddies, and its influence on the coastal water qualities. These environmental properties have been suggested to potentially influence the characteristics of seaweed bed distribution in the Tsushima Islands.

Effect of Ecosystem Degradation on the Source of Particulate Organic Matter in a Karst Lake: A Case Study of the Caohai Lake, China

The cycle of biogenic elements in lakes is intimately linked with particulate organic matter (POM), which plays a critical role in ecosystem restoration and the control of eutrophication. However, little is known regarding the functionality of ecosystem degradation on the source of POM in the water of a karst lake. To fill this knowledge gap, herein we compared the temporal and spatial distribution characteristics of POM prior to and after ecosystem degradation in the karst lake Caohai Lake, located in the southwest of China, and analyzed the source of POM using a combination of carbon and nitrogen stable isotopes (δ13C–δ15N). Our results showed that the dissolved oxygen (DO) concentration and pH values decreased, and the concentrations of POM in water increased by 11% and 31% in the wet and dry seasons, respectively. The decrease in the δ13C value of POM was accompanied by the increase in the δ15N value of POM in the water of Caohai lake. Prior to the ecosystem’s degradation, sediment resuspension (28%) and submerged macrophytes (33%) were the dominant sources of POM in lake water. In contrast, sediment resuspension (51%) was the major source of POM after the ecosystem’s degradation. Environmental factors, including DO, turbidity, water depth, and water temperature, that are related to photosynthesis and sediment resuspension are the main factors controlling the spatiotemporal distribution of POM. The resuspension of sediment reduced the transparency of the water, limiting effective photosynthesis, impeding the survival of submerged macrophytes, and, consequently, deteriorating the ecosystem. We propose that the control of sediment resuspension is important for improving the water transparency that creates an appropriate habitat for the restoration of the submerged macrophyte community.

Investigation of Deep-Sea Ecosystems Using Marker Fatty Acids: Sources of Essential Polyunsaturated Fatty Acids in Abyssal Megafauna.

Abyssal seafloor ecosystems cover more than 50% of the Earth’s surface. Being formed by mainly heterotrophic organisms, they depend on the flux of particulate organic matter (POM) photosynthetically produced in the surface layer of the ocean. As dead phytoplankton sinks from the euphotic to the abyssal zone, the trophic value of POM and the concentration of essential polyunsaturated fatty acids (PUFA) decrease. This results in pronounced food periodicity and limitations for bottom dwellers. Deep-sea invertebrate seston eaters and surface deposit feeders consume the sinking POM. Other invertebrates utilize different food items that have undergone a trophic upgrade, with PUFA synthesized from saturated and monounsaturated FA. Foraminifera and nematodes can synthesize arachidonic acid (AA), eicosapentaenoic acid (EPA), while some barophylic bacteria produce EPA and/or docosahexaenoic acid. FA analysis of deep-sea invertebrates has shown high levels of PUFA including, in particular, arachidonic acid, bacterial FA, and a vast number of new and uncommon fatty acids such as 21:4(n-7), 22:4(n-8), 23:4(n-9), and 22:5(n-5) characteristic of foraminifera. We suppose that bacteria growing on detritus having a low trophic value provide the first trophic upgrading of organic matter for foraminifera and nematodes. In turn, these metazoans perform the second-stage upgrading for megafauna invertebrates. Deep-sea megafauna, including major members of Echinodermata, Mollusca, and Polychaeta display FA markers characteristic of bacteria, foraminifera, and nematodes and reveal new markers in the food chain.

Changes in the characteristics of organic matter associated with hydrodynamics and phytoplankton size structure in the central-eastern Yellow Sea

The central-eastern Yellow Sea is an important region for transporting organic matter (OM) to the Pacific Ocean, however, there is limited information available regarding the characteristics and sources of OM in this area. The present study investigated the concentrations and stable isotopic compositions of carbon (δ13C) and nitrogen (δ15N) for particulate matter and sediment in the central-eastern Yellow Sea during April 2019. The physicochemical properties (i.e., salinity, temperature, fluorescence, and nutrients), size-fractionated phytoplankton biomass (Chl-a), and concentration and fluorescence characteristics of dissolved organic matter (DOM) were also determined. The satellite SST and Chl-a data indicated that mixing cold and warm water masses were observed. Phytoplankton blooms occurred a few days before our sampling campaign. Considering the high concentration of suspended solids in the bottom layer, resuspended sediment caused by tidal currents could be a major source of OM in coastal areas. The δ13C values of particulate organic matter (POM) in the coastal area were higher (−23 to −22‰) than those of OM from terrestrial sources (approximately −28 to −27‰). Instead, the lowest δ13C values were observed in the central part of our study area, where the relative abundance of picophytoplankton was high. These results indicated that phytoplankton-derived OM after phytoplankton spring blooms in the coastal area could be the primary source of OM rather than terrestrial origins. In addition, the source of OM that presented low δ13C values could be picophytoplankton-derived OM. The characteristics of DOM were related to biological processes (mediated by phytoplankton and bacteria) and resuspension of sedimentary organic matter. We did not detect an influx of large amounts of terrestrial OM in coastal sediments. Overall, the source and characteristics of OM appeared to be influenced by the hydrodynamics and the distribution properties of lower trophic-level organisms in the central-eastern Yellow Sea during the spring season.

Carbon and nitrogen isotope constraints on source and variation of particulate organic matter in high-latitude agricultural rivers, Northeast China

The release of biospheric organic matter (OM) stored in the humus-rich black soils of Northeast China is highly susceptible to permafrost thaw and agriculturally induced erosion processes, and plays a disproportionate role in the global carbon cycle. However, the perturbations of global change and anthropogenic activities on riverine export of particulate organic carbon (POC) in the high-latitude agricultural areas remain poorly constrained. In the present study, this knowledge gap was filled by identifying the provenances of particulate organic matter (POM) and the erosional flux of POC from the rivers draining Northeast China. The elemental (POC%, PON% and C/N ratio) and isotopic (δ13C and δ15N) compositions of POM in suspended particulate matter (SPM) from the rivers in Northeast China (the Songhua River (SRB) and Liao River basins (LRB)) were analyzed. The results showed that during the wet season, the δ13C and δ15N values of POM in SPM were −25.8 ± 1.2‰ and 6.6 ± 2.2‰ respectively. Elemental and isotopic characteristics of the SPM samples and potential sources indicated that the sources of POM differed significantly among different sampling periods and sites. During the wet season, Soil OM was the primary POM contributor. Compared to the SRB, autochthonous OM made relatively larger contributions to POM in the LRB. During the dry season, for the SPM samples mainly from the LRB, the δ13C and δ15N values of POM were −22.7 ± 4.3‰ and 3.1 ± 2.4% respectively, suggesting that effluent detritus was the predominant source. The flux and yield of SPM revealed the seasonal and spatial variations of physical erosion rate that controls POC export. Seasonally, up to 61% of the annual SPM load and 58% of the annual POC flux of the SRB occurred in May and August due to thawing processes and increasing precipitation. Spatially, the lower reaches of the SRB had the highest SPM yield and contributed more than half of the annual SPM load of the SRB, likely attributable to the high intensity of agricultural activities. Globally, the POC% in SPM and POC yield of the SRB were larger than a series of the world's largest rivers under a given SPM condition, suggesting the high sensitivity of terrestrial OM export to erosion. These findings highlight the perturbation and mobilization of terrestrial OM in Northeast China under intensive agricultural activities and ongoing global climate change.

Evaluating organic geochemical proxies for application to coastal lake sediments along the Gulf Coast of Florida for paleotempestology

Organic geochemical proxies (OGPs) preserved in coastal lake sediments have recently been used to reconstruct millennia-long hurricane records. Although OGPs (δ13C, δ15N and C/N) appear to be a more sensitive indicator of past storm events than traditional proxies used in paleotempestology, the method has not been tested in many lakes with modern data. In this study, we measured the δ13C, δ15N and C/N values of particulate organic matter (POM), along with the salinities and stable isotopic compositions of water (δ18O and δD), in two coastal lakes on seasonal or much shorter time scales throughout a 3-year period. This work was intended to develop a better understanding of the dynamics of OGPs in these lakes. The time-series data not only show that geochemical properties of these lakes varied seasonally, reflecting variations in lake biological and environmental conditions, but displayed unique variation patterns in response to storms that caused either seawater flooding or freshwater flooding of the lakes. The data show that seawater flooding led to higher δ13C and δ15N values, with either lower or no change in the C/N ratios of the POM, generally consistent with a previously proposed conceptual model for detecting seawater flooding events. The data also show that freshwater flooding reduces δ15N and increases the C/N values of POM, in addition to lowering the salinity, δ18O and δD of lake water. These time-series modern data demonstrate the feasibility of detecting past storm events that were large enough to cause either seawater or freshwater flooding by examining variation patterns of multiple OGPs in coastal lake sediments. Applying this understanding to a sediment core from one of the study lakes (Mullet Pond, Florida), we reconstructed a centuries-long record of storms based on the variation patterns in OGPs together with 210Pb ages. The OGP-based storm record suggests that 30 flooding events, including 15 seawater flooding and 15 freshwater flooding incidents, occurred over the last 165 years. Almost all of these flooding events can be matched (within the dating uncertainty) to the historic hurricanes that are known to have passed within 150 km of the study site. This further confirms that OGPs preserved in sediments in coastal lakes of Florida or similar regions may be used as reliable recorders of past storm activity.

Mercury biomagnification in a Southern Ocean food web.

Biomagnification of mercury (Hg) in the Scotia Sea food web of the Southern Ocean was examined using the stable isotope ratios of nitrogen (δ15N) and carbon (δ13C) as proxies for trophic level and feeding habitat, respectively. Total Hg and stable isotopes were measured in samples of particulate organic matter (POM), zooplankton, squid, myctophid fish, notothenioid fish and seabird tissues collected in two years (austral summers 2007/08 and 2016/17). Overall, there was extensive overlap in δ13C values across taxonomic groups suggesting similarities in habitats, with the exception of the seabirds, which showed some differences, possibly due to the type of tissue analysed (feathers instead of muscle). δ15N showed increasing enrichment across groups in the order POM to zooplankton to squid to myctophid fish to notothenioid fish to seabirds. There were significant differences in δ15N and δ13C values among species within taxonomic groups, reflecting inter-specific variation in diet. Hg concentrations increased with trophic level, with the lowest values in POM (0.0005±0.0002μgg-1 dw) and highest values in seabirds (3.88±2.41μgg-1 in chicks of brown skuas Stercorarius antarcticus). Hg concentrations tended to be lower in 2016/17 than in 2007/08 for mid-trophic level species (squid and fish), but the opposite was found for top predators (i.e. seabirds), which had higher levels in the 2016/17 samples. This may reflect an interannual shift in the Scotia Sea marine food web, caused by the reduced availability of a key prey species, Antarctic krill Euphausia superba. In 2016/17, seabirds would have been forced to feed on higher trophic-level prey, such as myctophids, that have higher Hg burdens. These results suggest that changes in the food web are likely to affect the pathway of mercury to Southern Ocean top predators.

Particulate organic matter release below melting sea ice (Terra Nova Bay, Ross Sea, Antarctica): Possible relationships with zooplankton

Timing and rates of release of particulate organic matter (POM) beneath the Antarctic sea ice during the melting season are relatively unknown. To shed light on this topic, we investigated: i) quantity and biochemical composition of POM released below annual sea ice in Terra Nova Bay (TNB, Ross Sea, Antarctica) through sediment traps deployed at 10- and 30-m depth; ii) the abundance and taxonomic composition of phytoplankton; iii) the abundance and biochemical composition of two zooplankton species, namely the pteropods Clione limacina antarctica and Limacina helicina antarctica. Overall, our results show that in late spring-early summer in TNB the melting of annual sea ice determines a rapid release of particles into the underlying water column, and that those particles are transported downward at the rate of hours to days. POM fluxes were dominated by lipids, which probably resulted from zooplankton grazing activity on sea-ice algae and faecal pellets that were released into the water column. This, after the initial injection of particles, which were likely associated with sinking sympagic microalgal biomass derived from the sea ice melting, enhanced the nutritional value of POM. POM released beneath the sea ice, in turn, had a possible effect on the biochemical composition of the two pteropod species. Our results confirm that melting of sea ice in spring-summer each year in Antarctic coastal waters represents a fundamental step in the transfer of energy towards the higher trophic levels. Evidence of a decline in Antarctic sea ice over the last five years, plausibly as a consequence of global warming, underlines a conspicuous decline in habitat availability for sea-ice algae and, as a consequence, a drop in the availability of food for pteropods and the higher trophic levels of the coastal Antarctic Ocean.

Trophic Structure and Sources of Variation Influencing the Stable Isotope Signatures of Meso- and Bathypelagic Micronekton Fishes

To better understand spatiotemporal variation in the trophic structure of deep-pelagic species, we examined the isotope values of particulate organic matter (POM) (isotopic baseline) and seven deep-pelagic fishes with similar diet compositions but contrasting vertical distributions across mesoscale features in the Gulf of Mexico (GOM) using stable isotope and amino acid compound-specific isotope analyses. Species examined included four migratory (Benthosema suborbitale, Lepidophanes guentheri, Melamphaes simus, Sigmops elongatus) and three non-migratory zooplanktivorous fishes (Argyropelecus hemigymnus, Cyclothone obscura, Sternoptyx pseudobscura). Isotopic values of POM increased with depth, with meso- and bathypelagic samples characterized by higher δ13C and δ15N values relative to epipelagic samples. Despite similar diet compositions, mean δ15N values of fishes spanned 3.43‰ resulting in mean trophic position estimates among species varying by 1.09 trophic levels. Interspecific differences in δ15N were driven by higher δ15N values in the non-migratory and deepest dwelling C. obscura (10.61‰) and lower δ15N values in the migratory and shallowest dwelling L. guentheri (7.18‰) and B. suborbitale (8.11‰). Similarly, fish δ15NsourceAA values were correlated with depth, with values found to be lowest in the migratory L. guentheri and B. suborbitale and highest in the non-migratory C. obscura. Our data suggests that depth-related trends in fish δ15N and δ15NsourceAA values are driven by shallower dwelling species feeding within epipelagic food webs supported by POM with lower δ15N values, while deeper dwelling, non-migratory species increasingly use food webs at depth supported by POM with elevated δ15N values. Horizontal isotopic variation was observed across a large mesoscale oceanographic feature (Loop Current) for POM and migratory and non-migratory species which were characterized by higher δ13C and lower δ15N values in the anticyclonic Loop Current relative to surrounding water masses. Our results demonstrate that isotopic values of POM can vary significantly over relatively small horizontal and vertical scales, and that baseline variation can be conserved in the signatures of higher-order consumers. By gaining a more thorough understanding of the sources contributing to isotopic variation of deep-pelagic fishes, this paper will inform the design and interpretation of future feeding studies in the pelagic realm and advances our knowledge of deep-pelagic food web structure.

Sources of nitrogen to stream food webs in tributaries of the Red River Valley, Manitoba

Lake Winnipeg has undergone rapid eutrophication over the last several decades with a large portion of the associated nutrient loads delivered via the Red River. Consequently, subcatchments of the Red River Valley (RRV) are priorities for nutrient management, an aim that requires identification of key nutrient sources to stream food webs. We identified the primary sources of nitrogen to food webs of rural streams of the RRV, as well as variation in nitrogen input between spring and summer. We measured δ15N of particulate organic matter (POM) and collector-gatherer invertebrates collected in spring and summer from 20 subcatchments, which exhibited a range of agricultural intensity and size of municipal wastewater lagoons. δ15N values of POM and collector-gatherer invertebrates were best predicted by the presence of wastewater treatment lagoons, with δ15N values increasing with the number of people served by lagoons in spring and summer. When present, wastewater contributed a greater proportion of nitrogen to stream food webs than agricultural sources. Waste sources also had a greater relative contribution to food webs in summer than spring. Despite wastewater lagoons releasing nitrogen in short-term, pulsed discharges, the influence of wastewater on food web nitrogen persisted from the summer release into the following spring. Based on the observed importance of wastewater as a source of nitrogen to stream food webs, we recommend management agencies consider additional actions to reduce nutrient losses from wastewater treatment facilities as well as agricultural lands to more effectively protect aquatic ecosystems.

Stable isotopes (δ13C, δ15N) seasonal changes in particulate organic matter and in different life stages of anchoveta (Engraulis ringens) in response to local and large scale oceanographic variations in north and central Chile

We compared the isotopic (δ13C, δ15N) signals in particulate organic matter (POM), small sized copepods, exogenous feeding larvae, and anchoveta (Engraulis ringens) eggs and adults, in relation to oceanographic characteristics during this species’ reproductive season (late winter – spring, 2016 and 2017) in two spawning zones along the Humboldt Current: northern (Iquique, 20°S) and central Chile (Talcahuano, 36°S). Both zones are subject to wind-driven coastal upwelling (most of the year in Iquique but seasonal in Talcahuano) and differ in freshwater input (higher in winter in Talcahuano and almost null in Iquique). In Talcahuano, lower precipitations and river fluxes occurred in winter 2016 compared with 2017. Our results show that, there are large scale processes that similarly affect the δ13C and δ15N signatures in different locations along the Humboldt Current (coastal upwelling), but also clear seasonal differences in the sources of δ13C and δ15N occur locally at the base of the pelagic trophic webs (POM). The seasonal difference in winter δ13C is largely due to the influx of terrigenous organic carbon into the Talcahuano coast, product of increased river discharges in normal winter years (2017, δ13C POM, winter = −24.78‰ vs spring = −19.16‰). Higher δ15N POM values occurred at the start of the upwelling season in spring in both locations during both years (Iquique 2016, δ15N: winter = 7.80‰ vs. spring = 10.65‰; Iquique 2017: winter = 10.57‰ vs. spring = 14.95‰) (Talcahuano 2016, δ15N: winter = 8.70‰ vs. spring = 9.07‰; Talcahuano 2017: winter = 6.44‰ vs. spring = 9.76‰). In 2017, δ15N values of small copepods and exogenous feeding larvae were also higher in spring than winter, but the δ15N values in adult anchovetas and their eggs were not, suggesting the seasonal differences in δ15N decrease as the trophic position increased. Within seasons, the estimated larval trophic positions were similar across locations (TP = 2.4–2.5) and were lower than those of adults (TP = 3.0–3.4) in Talcahuano in 2017. The overall results suggest that inter-annual and seasonal variations in large scale wind-driven oceanographic processes affect the δ13C and δ15N signatures at the base of the trophic web with different intensities at different locations and thereby induce variation in the isotopic signatures all along the trophic web but with different magnitude between trophic levels.

Sample acidification has a predictable effect on isotopic ratios of particulate organic matter along the Chilean coast.

Stable isotopes of carbon and nitrogen have proved to be valuable tools for researchers working across the different subfields of ecology. However, the chemical pretreatment of samples prior to analytical determination of stable isotope ratios can influence the results, and therefore conclusions regarding the ecology of the taxon or system under study. Here, we determined the effect of vapor acidification with concentrated HCl on the δ13 C and δ15 N values of particulate organic matter (POM), which are commonly used as baselines for studies of trophic ecology, or to understand oceanographic patterns. Samples of marine POM were obtained along a large-scale latitudinal gradient (ca 3000 km) along the Chilean coast, along with a range of oceanographic variables thought to potentially influence inorganic carbon at each sampling location. A random subset of 50 samples was divided into two parts: one acidified by HCl fumigation treatment, and the other acting as a control. We compared paired differences in δ13 C and δ15 N values measured by continuous flow isotope ratio mass spectrometry and used a model selection approach to examine which oceanographic factor best explained shifts in values following acid treatment. Acidification resulted in statistically significant reductions in both δ13 C and δ15 N values, but the effect was relatively small. The model that best explained the differences between acidified and non-acidified δ13 C values included depth, salinity and sea-surface temperature at the sampling point. A regression of acidified on non-acidified δ13 C values shows that the treatment effect was strongest on samples more depleted in 13 C. The differences between δ13 C and δ15 N values in acidified and non-acidified samples are linear and predictable. This implies that the nature of the POM and its possible alteration during the acid treatment are important factors that support the reliable determination of the values of δ13 C and δ15 N of POM.

Spatial and temporal shifts in the diet of the barnacle Amphibalanus eburneus within a subtropical estuary.

The success of many sessile invertebrates in marine benthic communities is linked to their ability to efficiently remove suspended organic matter from the surrounding water column. To investigate the diet of the barnacle Amphibalanus eburneus, a dominant suspension feeder within the Indian River Lagoon (IRL) of central Florida, we compared the stable isotopes ratios (δ13C and δ15N) of barnacle tissue to those of particulate organic matter (POM). Collections were carried out quarterly for a year from 29 permanent sites and at sites impacted by an Aureoumbra lagunensis bloom. δ13C and δ15N values of Amphibalanus eburneus varied across sites, but δ15N was more stable over time. There was a range of δ15N values of Amphibalanus eburneus tissue from 6.0‰ to 10.5‰ across sites. Because land-based sources such as sewage are generally enriched in 15N, this suggests a continuum of anthropogenic influence across sites in the IRL. Over 70% of the variation in δ15N values of Amphibalanus eburneus across sites was driven by the δ15N values of POM, supporting a generalist feeding strategy on available sources of suspended organic matter. The dominance of this generalist consumer in the IRL may be linked to its ability to consume spatially and temporally variable food resources derived from natural and anthropogenic sources, as well as Aureoumbra lagunensis cells. Generalist consumers such as Amphibalanus eburneus serve an important ecological role in this ecosystem and act as a sentinel species and recorder of local, site-specific isotopic baselines.

From the epipelagic zone to the abyss: Trophic structure at two seamounts in the subtropical and tropical Eastern Atlantic - Part I zooplankton and micronekton

Specific mechanisms, driving trophic interactions within the pelagic community may be highly variable in different seamount systems. This study investigated the trophic structure of zooplankton and micronekton above and around Ampère and Senghor, two shallow seamounts in the subtropical and tropical Eastern Atlantic, and over the adjacent abyssal plains. For the identification of food sources and trophic positions stable isotope ratios (δ13C and δ15N) were used. δ13C ranged from −24.7‰ to −15.0‰ and δ15N covered a total range of 0.9–15.9‰. Based on epipelagic particulate organic matter, zooplankton and micronekton usually occupied the 1st–3rd trophic level, including herbivorous, omnivorous and carnivorous taxa. δ13C and δ15N values were generally lower in zooplankton and micronekton of the subtropical waters as compared to the tropical region, due to the differing nutrient availability and phytoplankton communities. Correlations between δ13C and δ15N values of particulate organic matter, zooplankton, micronekton and benthopelagic fishes suggest a linear food chain based on a single energy source from primary production for Ampère Seamount, but no evidence was found for an autochthonus seamount production as compared to the open ocean reference site. Between Senghor Seamount and the open ocean δ13C signatures indicate that hydrodynamic effects at seamounts may modify the energy supply at times, but evidence for a seamount effect on the trophic structure of the pelagic communities was weak, which supports the assumption that seamount communities rely to a large extent on advected food sources.

Relationships between electron transport system (ETS) activity and particulate organic matter features in three areas of the Ross Sea (Antarctica)

Electron transport system activity (ETSa) and particulate organic matter (POM) concentrations and composition were measured in three areas of the continental shelf of the Ross Sea during summer 2014, in the framework of the Ross Sea Mesoscale Experiment (ROME) project. We aimed at testing whether in the epipelagic layer (0–200m) ETS showed different activity depending on the geographical position and on the different hydrological structures that characterised each area, as eddy and fronts, and whether the ETSa of the microplanktonic fraction depended on POM quantitative and qualitative features. ETSa showed differences between the three areas, but within each of them the different hydrological conditions did not influence significantly the respiration activity. ETSa displayed significant correlations with POM, especially in the offshore areas characterised by residual ice influence and by a mesoscale eddy structure. In these zones ETSa was enhanced by good trophic value of POM, i.e. showing dominance of proteins and PON over structural carbohydrates and POC, respectively. The role of the phytoplanktonic fraction in ETSa was higher in the eddy-influenced area, that showed significantly higher chlorophyll-a concentrations. On the other hand, in the area placed coastward, the relationships between ETSa and POM changed. High ETSa were found in the subsurface layer and down to 100m depth and were related to more refractory POM, whose utilization would require higher energy. Different ETSa-POM relationships were consistent with the anomalous phytoplanktonic bloom detected in the coastward area, characterised by Phaeocystis. Thus, the anomalies of the primary producers are reflected by changes in POM respiration and potential C utilization.