Simone R. Alin

Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere

Streams, rivers, lakes, and other inland waters are important agents in the coupling of biogeochemical cycles between continents, atmosphere, and oceans. The depiction of these roles in global-scale assessments of carbon (C) and other bioactive elements remains limited, yet recent findings suggest that C discharged to the oceans is only a fraction of that entering rivers from terrestrial ecosystems via soil respiration, leaching, chemical weathering, and physical erosion. Most of this C influx is returned to the atmosphere from inland waters as carbon dioxide (CO2) or buried in sedimentary deposits within impoundments, lakes, floodplains, and other wetlands. Carbon and mineral cycles are coupled by both erosion–deposition processes and chemical weathering, with the latter producing dissolved inorganic C and carbonate buffering capacity that strongly modulate downstream pH, biological production of calcium-carbonate shells, and CO2 outgassing in rivers, estuaries, and coastal zones. Human activities substantially affect all of these processes.

Climate change decreases aquatic ecosystem productivity of Lake Tanganyika, Africa

Although the effects of climate warming on the chemical and physical properties of lakes have been documented, biotic and ecosystem-scale responses to climate change have been only estimated or predicted by manipulations and models. Here we present evidence that climate warming is diminishing productivity in Lake Tanganyika, East Africa. This lake has historically supported a highly productive pelagic fishery that currently provides 25–40% of the animal protein supply for the populations of the surrounding countries. In parallel with regional warming patterns since the beginning of the twentieth century, a rise in surface-water temperature has increased the stability of the water column. A regional decrease in wind velocity has contributed to reduced mixing, decreasing deep-water nutrient upwelling and entrainment into surface waters. Carbon isotope records in sediment cores suggest that primary productivity may have decreased by about 20%, implying a roughly 30% decrease in fish yields. Our study provides evidence that the impact of regional effects of global climate change on aquatic ecosystem functions and services can be larger than that of local anthropogenic activity or overfishing.

Limacina helicina shell dissolution as an indicator of declining habitat suitability owing to ocean acidification in the California Current Ecosystem

Few studies to date have demonstrated widespread biological impacts of ocean acidification (OA) under conditions currently found in the natural environment. From a combined survey of physical and chemical water properties and biological sampling along the Washington–Oregon–California coast in August 2011, we show that large portions of the shelf waters are corrosive to pteropods in the natural environment. We show a strong positive correlation between the proportion of pteropod individuals with severe shell dissolution damage and the percentage of undersaturated water in the top 100 m with respect to aragonite. We found 53% of onshore individuals and 24% of offshore individuals on average to have severe dissolution damage. Relative to pre-industrial CO2 concentrations, the extent of undersaturated waters in the top 100 m of the water column has increased over sixfold along the California Current Ecosystem (CCE). We estimate that the incidence of severe pteropod shell dissolution owing to anthropogenic OA has doubled in near shore habitats since pre-industrial conditions across this region and is on track to triple by 2050. These results demonstrate that habitat suitability for pteropods in the coastal CCE is declining. The observed impacts represent a baseline for future observations towards understanding broader scale OA effects.

Lake-level history of Lake Tanganyika, East Africa, for the past 2500 years based on ostracode-inferred water-depth reconstruction

Assemblages of ostracodes from sediment cores illuminate lake-level history at decadal to centennial timescales during the late Holocene at Lake Tanganyika, East Africa. The ostracode-based lake-level curves for several cores resemble both each other and the only previously published lake-level record of comparable resolution for Lake Tanganyika during this interval, successfully reconstructing known highstands, improving the chronology of known lowstands, and contributing new information on late Holocene lake-level variability at this important tropical African location. In agreement with other late Holocene records from East Africa, the surface level at Lake Tanganyika reflects predominantly arid conditions throughout this interval, interrupted by relatively brief episodes of higher precipitation and lake level. The most pronounced lowstand in the record occurs at V200^0 BC, with other significant lowstands dating to the intervals V200^500 AD, V700^850 AD, the Medieval WarmPeriod (MWP; V1050^1250 AD at Lake Tanganyika), and the latter part of the Little Ice Age (LIA; V1550^1850 AD). The most important wet intervals in the lake-level record are centered on V500 AD, V1500 AD, and V1870 AD. The highstands and lowstands reported here for Lake Tanganyika appear to be fairly coherent with other records of rainfall throughout East Africa during the MWP and the LIA. Prior to the MWP, paleoclimate records are apparently less coherent, although this may be a reflection of the resolution and abundance of recent paleoclimatic data available for this climatically complex region.

Biogeochemical characterization of carbon sources in the Strickland and Fly rivers, Papua New Guinea

the organic and inorganic composition of particulate and dissolved carbon at several lowland sites in the Fly and Strickland rivers and on the Strickland River floodplain. Isotopic, elemental, and biomarker results suggest that organic carbon in the Strickland River was more degraded than in the Fly River, with a greater input of ancient organics from upland sources, and that aquatic production constituted a larger source in the Fly River. Radiocarbon results indicate that all carbon fractions were older in the Strickland than in the Fly and that Strickland floodplain sediments were also depleted in radiocarbon. Collectively, these results suggest that rivers of New Guinea export a comparable amount of particulate organic carbon to the Amazon, with a significant contribution from radiocarbon-depleted sources.

Terrigenous organic matter in sediments from the Fly River delta‐clinoform system (Papua New Guinea)

loadings (0.5–1.0 mg C m 2 ), although several samples from the outer topset region, an area of sediment bypass, were characterized by lower carbon loadings indicative of enhanced carbon losses. Overall, the organic matter in both surface and subsurface sediments appeared to have predominantly a terrigenous origin, with no evidence for dilution and/or replacement by marine carbon. The measured compositions were consistent with contributions from modern vascular plant detritus, aged soil organic matter, and very old or fossil organic matter devoid of recognizable biochemicals.

Seasonal Carbonate Chemistry Covariation with Temperature, Oxygen, and Salinity in a Fjord Estuary: Implications for the Design of Ocean Acidification Experiments

Carbonate chemistry variability is often poorly characterized in coastal regions and patterns of covariation with other biologically important variables such as temperature, oxygen concentration, and salinity are rarely evaluated. This absence of information hampers the design and interpretation of ocean acidification experiments that aim to characterize biological responses to future pCO2 levels relative to contemporary conditions. Here, we analyzed a large carbonate chemistry data set from Puget Sound, a fjord estuary on the U.S. west coast, and included measurements from three seasons (winter, summer, and fall). pCO2 exceeded the 2008–2011 mean atmospheric level (392 µatm) at all depths and seasons sampled except for the near-surface waters (< 10 m) in the summer. Further, undersaturated conditions with respect to the biogenic carbonate mineral aragonite were widespread (Ωar<1). We show that pCO2 values were relatively uniform throughout the water column and across regions in winter, enriched in subsurface waters in summer, and in the fall some values exceeded 2500 µatm in near-surface waters. Carbonate chemistry covaried to differing levels with temperature and oxygen depending primarily on season and secondarily on region. Salinity, which varied little (27 to 31), was weakly correlated with carbonate chemistry. We illustrate potential high-frequency changes in carbonate chemistry, temperature, and oxygen conditions experienced simultaneously by organisms in Puget Sound that undergo diel vertical migrations under present-day conditions. We used simple calculations to estimate future pCO2 and Ωar values experienced by diel vertical migrators based on an increase in atmospheric CO2. Given the potential for non-linear interactions between pCO2 and other abiotic variables on physiological and ecological processes, our results provide a basis for identifying control conditions in ocean acidification experiments for this region, but also highlight the wide range of carbonate chemistry conditions organisms may currently experience in this and similar coastal ecosystems.

Effects of land-use change on aquatic biodiversity: A view from the paleorecord at Lake Tanganyika, East Africa

Population growth and watershed deforestation in northwestern Tanzania threaten the biodiversity of Lake Tanganyika through erosion and habitat degradation. We used cores collected offshore from Gombe Stream National Park and a deforested watershed to reconstruct how land-use changes in the Gombe Stream area since A.D. 1750 have affected lake biodiversity. Paleoenvironmental and paleoecological data reveal substantial changes in mass accumulation rates for sediment and organic matter, nitrogen stable isotope values, and benthic species composition offshore from the deforested watershed since 1880. Comparable changes were not observed offshore from the park.

Estimating the Surface Area of Small Rivers in the Southwestern Amazon and Their Role in CO2 Outgassing

Abstract A recent estimate of CO2 outgassing from Amazonian wetlands suggests that an order of magnitude more CO2 leaves rivers through gas exchange with the atmosphere than is exported to the ocean as organic plus inorganic carbon. However, the contribution of smaller rivers is still poorly understood, mainly because of limitations in mapping their spatial extent. Considering that the largest extension of the Amazon River network is composed of small rivers, the authors’ objective was to elucidate their role in air–water CO2 exchange by developing a geographic information system (GIS)-based model to calculate the surface area covered by rivers with channels less than 100 m wide, combined with estimated CO2 outgassing rates at the Ji-Parana River basin, in the western Amazon. Estimated CO2 outgassing was the main carbon export pathway for this river basin, totaling 289 Gg C yr−1, about 2.4 times the amount of carbon exported as dissolved inorganic carbon (121 Gg C yr−1) and 1.6 times the dissolved organic...

Seasonal variability in the sources of particulate organic matter of the Mekong River as discerned by elemental and lignin analyses

[1] The Mekong River ranks within the top ten rivers of the world in terms of water discharge and sediment load to the ocean, yet its organic matter (OM) composition remains unstudied. This river is experiencing anthropogenically forced changes due to land use and impoundment, and these changes are expected to intensify in the future. Accordingly, we monitored the composition (including vascular-plant signatures) of Mekong River fine particulate organic matter (FPOM) over a one-year period. Autochthonous production comprises a greater proportion of FPOM during the dry season than in the rainy season, as demonstrated by higher percent organic carbon values (7.9 � 2.4 versus 2.2 � 0.4%), lower yields of lignin normalized to carbon (0.40 � 0.05 versus 1.1 � 0.3 mg (100 mg OC) � 1 ), and an increase in N:C ratios toward phytoplankton values during the dry season (from 0.06 to 0.12). Changes in the lignin-phenol composition of FPOM suggest that gymnosperms contribute more toward FPOM composition during the dry season, with angiosperms dominating in the wet season. This is supported by calculations of the lignin phenol vegetation index of riverine FPOM, which increases between the dry to wet seasons (dry: 29.4 � 15.0 versus wet: 74.6 � 17.3). These changes likely reflect seasonal differences in the proportion of flow that is coming from the Upper and Lower Basin, corresponding to compositional differences between the vegetation of these regions. Therefore, this work provides a baseline understanding of FPOM variability that can be used to assess how future change will affect this river.