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Shelled pteropods are widely regarded as bioindicators for ocean acidification, because their fragile aragonite shells are susceptible to increasing ocean acidity. While short-term incubations have demonstrated that pteropod calcification is negatively impacted by ocean acidification, we know little about net calcification in response to varying ocean conditions in natural populations. Here, we examine in situ calcification of Limacina helicina pteropods collected from the California Current Ecosystem, a coastal upwelling system with strong spatial gradients in ocean carbonate chemistry, dissolved oxygen and temperature. Depth-averaged pH ranged from 8.03 in warmer offshore waters to 7.77 in cold CO2-rich waters nearshore. Based on high-resolution micro-CT technology, we showed that shell thickness declined by ~ 37% along the upwelling gradient from offshore to nearshore water. Dissolution marks covered only ~ 2% of the shell surface area and were not associated with the observed variation in shell thickness. We thus infer that pteropods make thinner shells where upwelling brings more acidified and colder waters to the surface. Probably the thinner shells do not result from enhanced dissolution, but are due to a decline in calcification. Reduced calcification of pteropods is likely to have major ecological and biogeochemical implications for the cycling of calcium carbonate in the oceans. [ABSTRACT FROM AUTHOR]

Syntheses of carbonate chemistry spatial patterns are important for predicting ocean acidification impacts, but are lacking in coastal oceans. Here, we show that along the North American Atlantic and Gulf coasts the meridional distributions of dissolved inorganic carbon (DIC) and carbonate mineral saturation state (Ω) are controlled by partial equilibrium with the atmosphere resulting in relatively low DIC and high Ω in warm southern waters and the opposite in cold northern waters. However, pH and the partial pressure of CO2 (pCO2) do not exhibit a simple spatial pattern and are controlled by local physical and net biological processes which impede equilibrium with the atmosphere. Along the Pacific coast, upwelling brings subsurface waters with low Ω and pH to the surface where net biological production works to raise their values. Different temperature sensitivities of carbonate properties and different timescales of influencing processes lead to contrasting property distributions within and among margins. Anthropogenic CO2 is acidifying the ocean, but knowledge of the carbonate properties underlying these dynamics in coastal oceans is lacking. Here, the authors reveal spatial distribution patterns and variability in carbonate chemistry along North America's coasts. [ABSTRACT FROM AUTHOR]

Paleorecords from multiple indicators of environmental change provide evidence for the interactions between climate, human alteration of watersheds and lake ecosystem processes at Lake Tanganyika, Africa, a lake renowned for its extraordinary biodiversity, endemism and fisheries. This paper synthesizes geochronology, sedimentology, paleoecology, geochemistry and hydrology studies comparing the history of deltaic deposits from watersheds of various sizes and deforestation disturbance levels along the eastern coast of the lake in Tanzania and Burundi. Intersite differences are related to climate change, differences in the histories of forested vs. deforested watersheds, differences related to regional patterns of deforestation, and differences related to interactions of deforestation and climate effects. Climate change is linked to variations in sediment accumulation rates, charcoal accumulation, lake level and water chemistry, especially during the arid-humid fluctuations of the latter part of the Little Ice Age. Differences between forested and deforested watersheds are manifested by major increases in sediment accumulation rates in the latter (outside the range of climatically driven variability and for the last ∼40 years unprecedented in comparison with other records from the lake in the late Holocene), differences in eroded sediment and watershed stream composition, and compositional or diversity trends in lake faunal communities related to sediment inundation. Variability in regional patterns of deforestation is illustrated by the timing of transitions from numerous sedimentologic, paleoecologic and geochemical indicators. These data suggest that extensive watershed deforestation occurred as early as the late-18th to the early-19th centuries in the northern part of the Lake Tanganyika catchment, in the late-19th to early-20th centuries in the northern parts of modern-day Tanzania, and in the mid-20th century in central Tanzania. Rapid increases in sediment and charcoal accumulation rates, palynological and lake faunal changes occurred in the early-1960s. We interpret this to be the result of greatly enhanced flushing of sediments in previously deforested watersheds triggered by extraordinary rainfall in 1961/62. Regional differences in deforestation histories can be understood in light of the very different cultural and demographic histories of the northern and central parts of the lake shoreline. Incursion of slaving and ivory caravans from the Indian Ocean to the central coast of Lake Tanganyika by the early-19th century, with their attendant diseases, reduced human and elephant populations and therefore maintained forest cover in this region through the late-19th to early-20th centuries. In contrast, the northeastern portion of the lakeshore did not experience the effects of the caravan trades and consequently experienced high human population densities and widespread deforestation much earlier. These studies demonstrate the importance of paleolimnological data for making informed risk assessments of the potential effects of watershed deforestation on long-term lake ecosystem response in the Lake Tanganyika catchment. Differences in sediment yield and lake floor distribution of that yield, linked to factors such as watershed size, slope, and sediment retention, must be accounted for in management plans for both human occupation of currently forested watersheds and the development of future underwater reserves. [ABSTRACT FROM AUTHOR]

We investigated paleolimnological records from a series of river deltas around the northeastern rim of Lake Tanganyika, East Africa (Tanzania and Burundi) in order to understand the history of anthropogenic activity in the lake’s catchment over the last several centuries, and to determine the impact of these activities on the biodiversity of littoral and sublittoral lake communities. Sediment pollution caused by increased rates of soil erosion in deforested watersheds has caused significant changes in aquatic communities along much of the lake’s shoreline. We analyzed the effects of sediment discharge on biodiversity around six deltas or delta complexes on the east coast of Lake Tanganyika: the Lubulungu River delta, Kabesi River delta, Nyasanga/Kahama River deltas, and Mwamgongo River delta in Tanzania; and the Nyamuseni River delta and Karonge/Kirasa River deltas in Burundi. Collectively, these deltas and their associated rivers were chosen to represent a spectrum of drainage-basin sizes and disturbance levels. By comparing deltas that are similar in watershed attributes (other than disturbance levels), our goal was to explore a series of historical “experiments” at the watershed scale, with which we could more clearly evaluate hypotheses of land use or other effects on nearshore ecosystems. Here we discuss these deltas, their geologic and physiographic characteristics, and the field procedures used for coring and sampling the deltas, and various indicators of anthropogenic impact. [ABSTRACT FROM AUTHOR]

Fossil invertebrates from cores collected in Lake Tanganyika provide a record of probable nearshore ecological response to recent watershed deforestation and sediment erosion in several East African watersheds. We compared paleoecological profiles (primarily from ostracodes) from watersheds spanning a range of sizes and present-day deforestation disturbance levels to understand the timing and magnitude of faunal changes, and their relationship in time to terrestrially-derived disturbance indicators. Profiles from the Lubulungu and Nyasanga/Kahama Rivers (Tanzania) provide a record of faunal variability in watersheds that are currently undisturbed with respect to deforestation. These records indicate continuous faunal turnover through time. However, this pattern of turnover is accompanied by relatively high levels of diversity throughout the record, with no wholesale extinction events. Ostracode taphonomic data and other fossil abundance data from the Lubulungu area provide strong evidence in support of at least two episodes of lower lake levels, associated with episodes of Late Holocene aridity. Records from deltas of disturbed watersheds at the Kabesi River (Tanzania) and those of Northern Burundi all show a combination of profound and abrupt faunal turnover, in some cases accompanied by local extinction and establishment of a few dominant taxa. At the Mwamgongo River delta, fed from a very small, disturbed watershed, species turnover was subtler. In disturbed watershed cores showing abrupt faunal changes the transitions mostly occurred in the late 19th to early 20th centuries, predating the major mid-20th century increase in sediment mass accumulation rates, with the latter only correlated with changing fossil abundance and flux. However, the earlier faunal community changes are contemporaneous with both palynological and geochemical changes in the core profiles indicative of changing land-use patterns. This suggests that lacustrine ecosystem response to deforestation was a two-stage process, with an earlier phase of response to changing quality of sediments or dissolved matter being discharged from the watershed, and a subsequent phase responding to increased quantity of sediment. [ABSTRACT FROM AUTHOR]

We established sediment geochronologies for cores from eight deltaic areas in Lake Tanganyika (the Lubulungu, Kabesi, Halembe, Malagarasi, Nyasanga/Kahama, Mwamgongo, Nyamusenyi, and Karonge/Kirasa River deltas), recording a range of watershed disturbance histories from the eastern margin of this African rift valley lake. Cores from currently disturbed sites on the central Tanzanian coast display remarkably uniform and low rates of sediment accumulation from the 18th century until the early 1960s, when a synchronous and dramatic rise in rates occurs. Through this same time interval sedimentation rates offshore from undisturbed Tanzanian watersheds either remain unchanged or decline. Further north, at disturbed sites along the northern Tanzania and Burundi coasts, the pattern of sedimentation rate increase is more complex. Although a mid-late 20th century increase is also evident in these sites, indications of earlier periods of increasing sediment erosion, dating from the mid-late 19th century, are also evident. Synchronous changes in sediment accumulation rates dating from the early 1960s may be the result of exceptionally wet years triggering an increase in the discharge of previously eroded and unconsolidated alluvium and stream/beach terrace deposits, previously accumulated in the deltas and stream valleys of impacted watersheds. Sedimentation rate impacts of deforestation on lake ecosystems are likely modulated by short-term climatic forcing events, which can impact the specific timing and location of sediment discharge to lakes. [ABSTRACT FROM AUTHOR]