Terrestrial Particulate Organic Carbon Research Articles

Phytoplankton, not allochthonous carbon, sustains herbivorous zooplankton production

Terrestrial organic matter inputs have long been thought to play an important role in aquatic food web dynamics. Results from recent whole lake (13)C addition experiments suggest terrestrial particulate organic carbon (t-POC) inputs account for a disproportionate portion of zooplankton production. For example, several studies concluded that although t-POC only represented approximately 20% of the flux of particulate carbon available to herbivorous zooplankton, this food source accounted for approximately 50% of the C incorporated by zooplankton. We tested the direct dietary impact of t-POC (from the leaves of riparian vegetation) and various phytoplankton on Daphnia magna somatic growth, reproduction, growth efficiency, and lipid composition. By itself, t-POC was a very poor quality resource compared to cryptophytes, diatoms, and chlorophytes, but t-POC had similar food quality compared to cyanobacteria. Small additions of high quality Cryptomonas ozolinii to t-POC-dominated diets greatly increased Daphnia growth and reproduction. When offered alone, t-POC resulted in a Daphnia growth efficiency of 5 +/- 1%, whereas 100% Cryptomonas and Scenedesmus obliquus diets resulted in growth efficiencies of 46 +/- 8% (+/- SD) and 36 +/- 3%, respectively. When offered in a 50:50 mixed diet with Cryptomonas or Scenedesmus, the t-POC fraction resulted in a partial growth efficiency of 22 +/- 9% and 15 +/- 6%, respectively. Daphnia that obtained 80% of their available food from t-POC assimilated 84% of their fatty acids from the phytoplankton component of their diet. Overall, our results suggest Daphnia selectively allocate phytoplankton-derived POC and lipids to enhance somatic growth and reproduction, while t-POC makes a minor contribution to zooplankton production.

Dispersal and transformation of organic carbon across an episodic, high sediment discharge continental margin, Waipaoa sedimentary system, New Zealand

The rivers that drain active, collisional margins deliver ∼ 35% of particulate organic carbon (POC) to the world ocean, and are a key component of the global organic carbon flux. However, knowledge of the fate of terrestrial POC in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets. Here, the fate of terrestrial POC is determined off the high discharge, Waipaoa River (sediment yield 15 Mt y − 1 ) based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope. Total organic carbon (%TOC), δ 13C, 14C, C/N ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink, and how these characteristics vary with river discharge. Complementary to surface sediments, down-core characteristics of three multi-cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods. The presence of flood deposits allows us to compare their OC characteristics with non-flood sediment, thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial OC through the marine environment. Results from this study show that as surface sediments are physically and biologically processed across the continental margin, they gain a marine signature. Biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore. Biomarkers also demonstrate that terrestrial OC is being transferred across the continental margin, with plant sterols, long chain alcohols and long chain fatty acids (biomarkers indicative of vascular plants) persisting as far offshore as the mid-continental slope. In contrast to ambient conditions represented by surface sediments, rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment. A 1–10 cm thick flood layer preserved from Cyclone Bola (March 1988) contains a significant amount of terrestrially-sourced OC which subsequently was rapidly buried by sediments delivered during less extreme conditions.

Dispersal and transformation of organic carbon across an episodic, high sediment discharge continental margin, Waipaoa Sedimentary System, New Zealand

The rivers that drain active, collisional margins of the southwest Pacific deliver up to 35% of particulate organic carbon (POC) to the world ocean, and are a key component of the global organic carbon flux. However, knowledge of the fate of terrestrial POC in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets. Here, the fate of terrestrial POC is determined off the high discharge, Waipaoa River (sediment yield 15 Mt y − 1 ) based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope. Total organic carbon (%TOC), δ 13C, 14C, C/N ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink, and how these characteristics vary with river discharge. Complementary to surface sediments, down-core characteristics of three multi-cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods. The presence of flood deposits allows us to compare their OC characteristics with non-flood sediment, thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial OC through the marine environment. Results from this study show that as surface sediments are physically and biologically processed across the continental margin, they gain a marine signature. Biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore. Biomarkers also demonstrate that terrestrial OC is being transferred across the continental margin, with plant sterols, long-chain alcohols and long-chain fatty acids (biomarkers indicative of vascular plants) persisting as far offshore as the mid-continental slope. In contrast to ambient conditions represented by surface sediments, rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment. A 1–10 cm thick flood layer preserved from Cyclone Bola (March 1988) contains a significant amount of terrestrially-sourced OC which subsequently was rapidly buried by sediments delivered during less extreme conditions.

Differential transport and degradation of bulk organic carbon and specific terrestrial biomarkers in the surface waters of a sub-arctic brackish bay mixing zone

Detailed organic geochemical analyses were performed on surface water particulate samples of the lower Kalix River and northern Bothnian Bay collected during the spring flood of 2005. Both bulk geochemical and molecular biomarker analyses indicated a predominance of terrestrially-derived particulate organic matter (POM), both of higher plant and Sphagnum origin in the low salinity zone (LSZ) of the Kalix River estuary, with an increasing contribution of marine-derived POM in the offshore Bothnian Bay basin. Two-dimensional box modeling of the mixed surface layer in the LSZ indicated that 65% of the particulate organic carbon (POC) and between 73 and 93% of the terrestrial biomarker classes analyzed (high molecular weight n-alkanes, n-alkanoic acids and n-alkanols as well as sitosterol) were degraded in the course of their weeklong transit through the inner LSZ during the spring flood. This corresponds to field-based degradation rate constants for the biomarkers of 0.5 and 2.5 day − 1 , which are similar to results reported from mesocosm experiments for related compounds. The degradation rate constant for terrestrial POC of 0.38 day − 1 was about 20 times larger than for DOC and suggests that POC mineralization stands for 44% of the total mineralization, which is much larger than previously considered. This sub-arctic river-export regime has a geochemistry resembling that of neighboring western Russian Arctic Rivers, suggesting that a large part of the OM coastally exported from northernmost Eurasian soils may be degraded within the vicinity of the river mouths and putatively be released as carbon dioxide. The 65% degradation of terrestrial POC in the coastal surface water of this sub-arctic recipient is substantially larger than a global-average of 35% used in recent budget estimates of the fate of terrestrially-exported POC on the pan-arctic shelves. Considering ongoing and predicted changes in the Arctic Region due to global warming a more efficient degradation of river-exported terrestrial POC may have far-reaching consequences for the large-scale biogeochemical cycling of carbon in the pan-arctic region and beyond.

Airborne carbon deposition on a remote forested lake

Airborne inputs of terrestrial particulate organic carbon (TPOC) were measured during summ- er stratification for an oligotrophic north temperate lake located in a forested watershed. These inputs were episodic and associated with wind and rain events. The rate of deposition decreased exponentially with distance from shore. Yet, about 55 % of the total airborne TPOC input occurred more than 12 m from shore on this 25.8 ha lake. Of total deposition, 39 % was less than 153 mm in diameter - a size fraction not commonly measured in prior studies. Average air- borne deposition was 5 mg C m -2 d -1 , which is consistent with measurements from other lakes and equivalent to about 1.1 % of daily net primary production in our study lake. C:N ratios of TPOC were between 6:1 and 22:1 (molar), much lower than the values for terrestrial leaves which were between 39:1 and 48:1 (molar). These low C:N ratios suggest that TPOC may be a useful substrate for aquatic consumers and may supplement in-lake primary production as a food source, especially after wind and rain events when airborne TPOC inputs are high.

Terrestrial and marine POC fluxes derived from 234Th distributions and δ13C measurements on the Mackenzie Shelf

Water column deficits of 234Th relative to 238U in the Mackenzie Shelf, Cape Bathurst Polynya, and Amundsen Gulf were used to estimate sinking fluxes of particulate organic carbon (POC) in these areas. The 234Th fluxes were converted to marine and terrestrial POC fluxes using the POC/Th ratio on filterable particles >70 μm and δ13C measurements to determine the fraction of marine and terrestrial POC. In June/July 2004, the greatest 234Th deficits (0–100 m: 56–95 dpm m−2) were observed in the Mackenzie outer shelf. Deficits in the upper 100 m ranged from 3 to 59 dpm m−2 in the Cape Bathurst Polynya. The δ13C values of POC in the >70‐μm particles filtered in situ pumps ranged from −25.1‰ to −28‰. Using a two‐end‐member mixing model with marine POC = −21.4‰ and terrestrial POC = −28‰ shows that terrestrial POC is most evident at the Mackenzie Shelf stations but is present throughout the region. The fraction of marine POC ranged from 0 to 59% in the area in June/July 2004, with highest values in the Cape Bathurst Polynya. Fluxes of marine POC in the polynya average ∼5 mmol C m−2 d−1 at 50 m in June 2004 and increase to ∼12 mmol C m−2 d−1 in July. Comparable fluxes are observed at 100 m in June, but values decrease to ∼6 mmol C m−2 d−1 at 100 m in July. These fluxes are greater than estimates of organic carbon remineralization and burial in sediments of the polynya (∼3 mmol m−2 d−1), suggesting that POC may be exported out of the area, effectively remineralized by microbial activity in the twilight zone or incorporated into biomass.

Differential support of lake food webs by three types of terrestrial organic carbon

Organic carbon inputs from outside of ecosystem boundaries potentially subsidize recipient food webs. Four whole-lake additions of dissolved inorganic 13C were made to reveal the pathways of subsidies to lakes from terrestrial dissolved organic carbon (t-DOC), terrestrial particulate organic carbon (t-POC) and terrestrial prey items. Terrestrial DOC, the largest input, was a major subsidy of pelagic bacterial respiration, but little of this bacterial C was passed up the food web. Zooplankton received <2% of their C from the t-DOC to bacteria pathway. Terrestrial POC significantly subsidized the production of both zooplankton and benthic invertebrates, and was passed up the food web to Chaoborus and fishes. This route supplied 33-73% of carbon flow to zooplankton and 20-50% to fishes in non-fertilized lakes. Terrestrial prey, by far the smallest input, provided some fishes with >20% of their carbon. The results show that impacts of cross-ecosystem subsidies depend on characteristics of the imported material, the route of entry into the food web, the types of consumers present, and the productivity of the recipient system.

Sources and transport of dissolved and particulate organic carbon in the Mississippi River estuary and adjacent coastal waters of the northern Gulf of Mexico

Dissolved organic carbon (DOC), stable carbon isotopic ( δ 13C) compositions of DOC and particulate organic carbon (POC), and elemental C/N ratios of POC were measured for samples collected from the lower Mississippi and Atchafalaya rivers and adjacent coastal waters in the northern Gulf of Mexico during the low flow season in June 2000 and high flow season in April 2001. These isotopic and C/N results combined with DOC measurements were used to assess the sources and transport of terrestrial organic matter from the Mississippi and Atchafalaya rivers to the coastal region in the northern Gulf of Mexico. δ 13C values of both POC (−23.8‰ to −26.8‰) and DOC (−25.0‰ to −29.0‰) carried by the two rivers were more depleted than the values measured for the samples collected in the offshore waters. Strong seasonal variations in δ 13C distributions were observed for both POC and DOC in the surface waters of the region. Fresh water discharge and horizontal mixing played important roles in the distribution and transport of terrestrial POC and DOC offshore. Our results indicate that both POC and DOC exhibited non-conservative behavior during the mixing especially in the mid-salinity range. Based on a simple two end-member mixing model, the comparison of the measured DOC- δ 13C with the calculated conservative isotopic mixing curve indicated that there was a significant in situ production of marine-derived DOC in the mid- to high-salinity waters consistent with our in situ chlorophyll- a measurements. Our DOC- δ 13C data suggest that a removal of terrestrial DOC mainly occurred in the high-salinity (>25) waters during the mixing. Our study indicates that the mid- to high- (10–30) salinity range was the most dynamic zone for organic carbon transport and cycling in the Mississippi River estuary. Variability in isotopic and elemental compositions along with variability in DOC and POC concentrations suggest that autochthonous production, bacterial utilization, and photo-oxidation could all play important roles in regulating and removing terrestrial DOC in the northern Gulf of Mexico and further study of these individual processes is warranted.

Riverine export of aged terrestrial organic matter to the North Atlantic Ocean.

Global riverine discharge of organic matter represents a substantial source of terrestrial dissolved and particulate organic carbon to the oceans. This input from rivers is, by itself, more than large enough to account for the apparent steady-state replacement times of 4,00-6,000 yr for oceanic dissolved organic carbon. But paradoxically, terrestrial organic matter, derived from land plants, is not detected in seawater and sediments in quantities that correspond to its inputs. Here we present natural 14C and 13C data from four rivers that discharge to the western North Atlantic Ocean and find that these rivers are sources of old (14C-depleted) and young (14C-enriched) terrestrial dissolved organic carbon, and of predominantly old terrestrial particulate organic carbon. These findings contrast with limited earlier data that suggested terrestrial organic matter transported by rivers might be generally enriched in 14C from nuclear testing, and hence newly produced. We also find that much of the young dissolved organic carbon can be selectively degraded over the residence times of river and coastal waters, leaving an even older and more refractory component for oceanic export. Thus, pre-ageing and degradation may alter significantly the structure, distributions and quantities of terrestrial organic matter before its delivery to the oceans.

Deposition and modification of a flood layer on the northern California shelf: lessons from and about the fate of terrestrial particulate organic carbon

The 1995 flood of the Eel River in northern California provided an opportunity to follow the short-term history of riverine particulate matter on a continental shelf. Particulate organic carbon, including both vascular plant debris and soil carbon, was utilized as a tracer of the distribution and modification of the shelf flood deposit. These components were sorted during initial emplacement of the deposit, both in the river plume and in the benthic boundary layer. During the 8 months following the flood, lateral transport, shallow burial, and bioturbation contributed to spatial changes in the amount and character of carbon in the flood deposit. Study of the deposit suggests that this and previous flood layers may be preserved in the shelf and upper slope stratigraphic record as clay-rich beds with relatively high carbon to nitrogen ratios and more negative δ 13C values than background, non-flood sediment. Concentrations of vascular plant debris are locally characteristic of such layers, and may serve as distinctive stratigraphic markers. Investigation of organic carbon in the 1995 flood layer on the Eel River shelf provides insight into the fate of terrestrial particulate carbon on continental margins in general. Flooding of the Eel River, and of similar river systems draining mountainous terrains, introduces large quantities of terrestrial carbon to the marine environment and may favor the preservation of both terrestrial and marine carbon. The abundant woody vascular plant debris discharged during floods is especially likely to survive oxidation and sulfate reduction in the shallower zones of the seabed. This study reveals the dominant role that physical and biological processes may play in attenuating the terrestrial organic-carbon signal on shelves over monthly to yearly time scales.

Particulate organic carbon supply to the sea bottom: stable carbon isotope ratio analysis of the sediment trap samples at the mouth of otsuchi bay, Northeastern Japan

An array of sediment traps was deployed for the analysis of the pattern of particulate organic carbon (POC) supply to the sea bottom in April, May and July 1988 at the mouth of Otsuchi Bay (about 80 m depth), Northeastern Japan. On the basis of a simple two-component mixing model using stable carbon isotope ratios, the POC flux was separated into marine planktonic and terrestrial components. Both the planktonic and terrestrial POC fluxes had maximum values at 30 m above the sea bottom throughout the three experiments. The planktonic POC flux showed a significant decrease with depth between 30 m and 10 m or 5 m above the bottom. Vertical supply of the planktonic POC and supply of the resuspended planktonic POC were estimated on the basis of regression lines between water depth and the planktonic POC flux in the depth range where the flux decreases with depth. Vertical supply of the planktonic POC and supply of the resuspended planktonic POC to the sea bottom were largest in May (52.1 mgC m−2 d−1 and 19.5 mgC m−2 d−1 at 5 m above the sea bottom), and horizontal supplies of the terrestrial POC were almost constant (31.9±3.5 mgC m−2 d−1 at 5 m above the bottom) throughout the three experiments.