Sedimentology and geochemistry of the sand fraction in surface sediments off West Africa

Surface sediments from 5 profiles between 30 and 3000 m water depth off W Africa (12-19° N) have been studied for their sand fraction composition and their total calcium carbonate and organic matter contents to evaluate the effect of climatic and hydrographic factors on actual sedimentation. On the shelf and upper slope (< 500 m), currents prevent the deposition of significant amounts of fine-grained material. The sediments forming here are characterized by high sand contents (> 60 %; in most samples > 89 %), low organic carbon contents (in most samples < 0.8 %), high median diameters of the sand fraction (120-500 µm), and by a predominance of quartz and biogenic relict shells (most abundant: molluscs and bryozoans) in the sand fraction. Median diameters of total sand fraction and of major biogenic sand fraction components (biogenic relict material, benthonic molluscs, benthonic and planktonic foraminifers) co-vary to some extent and show maximum values in 100-300 m water depth, reflectingthe sorting effect of currents (perhaps the northward flowing undercurrent). In this water depth, biogenic relict material is considerably enriched relative to wuartz, the second dominating sand fraction component on the shelf and upper slope, resulting in distinct calcium carbonate maxima of the bulk sediments. The influence of the undercurrent is also reflected in a northward transport of fine grained river load and perhaps in the distribution of the red stained, coarse silt and sand-size clay aggregates, which show maxima in 300-500 m water depth. They probably originate from tropical soils. Abundant coarse red-stained quartz on the shelf off Cape Roxo (12-130° N) suggests a southward extension of last glacial dune fields to this latitude. Below about 500 m water depth, current influence becomes negligible - as indicated by a strong decrease in sand content, a concomitant increase in sedimentary organic carbon contents (up to 2.5-3.5 %), and the occurence of high mica/quartz ratios in the sand fraction. Downslope transport, presumably due to the bioturbation mechanism, is indicated by the presence of coarse shelf-borne particles (glauconite, relict shells) down to about 1000 m water depth. The fine/coarse ratio (clay + silt/sand) of the sediments from water deoth > 500 m never exceed a value of 11 in northern latitudes (19° - 26° N), but shows distinct maxima, ranging from 50 to 120, at latitudes 18°, 17° 15°30', and 14° N in about 2000 m water depth. This distribution is attributed to the deposition of fine-grained river load at the continental slope between 18° and 14° N, brought into the sea by the Senegal and souther rivers and transported northward ny the undercurrent. Strong calcium carbonate dissolution is indicated by the complete disappearance of pteropodes (aragonite) and high fragmentation of the planktoic foraminifers (calcite) in sediments from water depth > 300-600 m. Fragmentation ratios of planktonic foraminifers were found to depend on the organic carbon/carbonate ratios of the sediment suggesting that calcite dissolution at the sea bottom may also be significant in shelf and continental slope water depths if the organic matter/carbonate ratio of the surface sediment is high and the test remain long enough within the oxidizing layer on the top of the sulfate reduction zone. The fact that in the region under study intensity and anual duration of upwelling decrease from north to south is neither reflected in the composition on the sand fraction (i.e. radiolarian and fish debris contents, radiolarian/planktonic foraminiferal ratios, benthos/plankton ratios of foraminifers), nor in the sedimentary organic carbon distribution. On the contrary, these parameters even show in comparable water depths a tendency for highest values in the south, partly because primary production rates remain high in the whole region, particularly on the shelf, due to the nutrient input by rivers in the south. In addition, several hydrographic, sedimentological and climatic factors severely affect their distribution - for example currents, dissolution, grain size composition, deposition of river load, and bulk sedimentation rats.

Specific microbial biomarker signatures from Congo fan sediments of ODP site 175-1075, Congo soils, wetland and estuary sediments

Methane (CH4) is a strong greenhouse gas known to have perturbed global climate in the past, especially when released in large quantities over short time periods from continental or marine sources. It is therefore crucial to understand and, if possible, quantify the individual and combined response of these variable methane sources to natural climate variability. However, past changes in the stability of greenhouse gas reservoirs remain uncertain and poorly constrained by geological evidence. Here, we present a record from the Congo fan of a highly specific bacteriohopanepolyol (BHP) biomarker for aerobic methane oxidation (AMO), 35-aminobacteriohopane-30,31,32,33,34-pentol (aminopentol), that identifies discrete periods of increased AMO as far back as 1.2 Ma. Fluctuations in the concentration of aminopentol, and other 35-aminoBHPs, follow a pattern that correlates with late Quaternary glacial-interglacial climate cycles, with highest concentrations during warm periods. We discuss possible sources of aminopentol, and the methane consumed by the precursor methanotrophs, within the context of the Congo River setting, including supply of methane oxidation markers from terrestrial watersheds and/or marine sources (gas hydrate and/or deep subsurface gas reservoir). Compound-specific carbon isotope values of -30 per mil to -40 per mil for BHPs in ODP 1075 and strong similarities between the BHP signature of the core and surface sediments from the Congo estuary and floodplain wetlands from the interior of the Congo River Basin, support a methanotrophic and likely terrigenous origin of the 35-aminoBHPs found in the fan sediments. This new evidence supports a causal connection between marine sediment BHP records of tropical deep sea fans and wetland settings in the feeding river catchments, and thus tropical continental hydrology. Further research is needed to better constrain the different sources and pathways of methane emission. However, this study identifies the large potential of aminoBHPs, in particular aminopentol, to trace and, once better calibrated and understood, quantify past methane sources and fluxes from terrestrial and potentially also marine sources.

Avian species identity and predation success in tropical cacao agroforestry of the Napu Valley in Central Sulawesi, Indonesia

Avian ecosystem services such as the suppression of pests are considered being of high ecological and economic importance in a range of ecosystems, especially in tropical agroforestry. But how bird predation success is related to the diversity and composition of the bird community, as well as local and landscape factors, is poorly understood. The author quantified arthropod predation in relation to the identity and diversity of insectivorous birds, using experimental exposure of artificial, caterpillar-like prey on smallholder cacao agroforestry systems, differing in local shade management and distance to primary forest. The bird community was assessed using both mist netting (targeting on active understory insectivores) and point count (higher completeness of species inventories) sampling. The study was conducted in a land use dominated area in Central Sulawesi, Indonesia, adjacent to the Lore Lindu National Park. We selected 15 smallholder cacao plantations as sites for bird and bat exclosure experiments in March 2010. Until July 2011, we recorded several data in this study area, including the bird community data, cacao tree data and bird predation experiments that are presented here. We found that avian predation success can be driven by single and abundant insectivorous species, rather than by overall bird species richness. Forest proximity was important for enhancing the density of this key species, but did also promote bird species richness. The availability of local shade trees had no effects on the local bird community or avian predation success. Our findings are both of economical as well as ecological interest because the conservation of nearby forest remnants will likely benefit human needs and biodiversity conservation alike.

Combined effects of CO2 and light on large and small isolates of the unicellular N2-fixing cyanobacterium Crocosphaera watsonii from the western tropical Atlantic Ocean

We examined the combined effects of light and pCO2 on growth, CO2-fixation and N2-fixation rates by strains of the unicellular marine N2-fixing cyanobacterium Crocosphaera watsonii with small (WH0401) and large (WH0402) cells that were isolated from the western tropical Atlantic Ocean. In low-pCO2-acclimated cultures (190 ppm) of WH0401, growth, CO2-fixation and N2-fixation rates were significantly lower than those in cultures acclimated to higher (present-day 385 ppm, or future 750 ppm) pCO2 treatments. Growth rates were not significantly different, however, in low-pCO2-acclimated cultures of WH0402 in comparison with higher pCO2 treatments. Unlike previous reports for C. watsonii (strain WH8501), N2-fixation rates did not increase further in cultures of WH0401 or WH0402 when acclimated to 750 ppm relative to those maintained at present-day pCO2. Both light and pCO2 had a significant negative effect on gross : net N2-fixation rates in WH0402 and trends were similar in WH0401, implying that retention of fixed N was enhanced under elevated light and pCO2. These data, along with previously reported results, suggest that C. watsonii may have wide-ranging, strain-specific responses to changing light and pCO2, emphasizing the need for examining the effects of global change on a range of isolates within this biogeochemically important genus. In general, however, our data suggest that cellular N retention and CO2-fixation rates of C. watsonii may be positively affected by elevated light and pCO2 within the next 100 years, potentially increasing trophic transfer efficiency of C and N and thereby facilitating uptake of atmospheric carbon by the marine biota.

Responses of three tropical seagrass species to CO2 enrichment

Increased atmospheric carbon dioxide leads to ocean acidification and carbon dioxide (CO2) enrichment of seawater. Given the important ecological functions of seagrass meadows, understanding their responses to CO2 will be critical for the management of coastal ecosystems. This study examined the physiological responses of three tropical seagrasses to a range of seawater pCO2 levels in a laboratory. Cymodocea serrulata, Halodule uninervis and Thalassia hemprichii were exposed to four different pCO2 treatments (442-1204 µatm) for 2 weeks, approximating the range of end-of-century emission scenarios. Photosynthetic responses were quantified using optode-based oxygen flux measurements. Across all three species, net productivity and energetic surplus (PG:R) significantly increased with a rise in pCO2 (linear models, P < 0.05). Photosynthesis-irradiance curve-derived photosynthetic parameters-maximum photosynthetic rates (P max) and efficiency (alpha) also increased as pCO2 increased (linear models, P < 0.05). The response for productivity measures was similar across species, i.e. similar slopes in linear models. A decrease in compensation light requirement (Ec) with increasing pCO2 was evident in C. serrulata and H. uninervis, but not in T. hemprichii. Despite higher productivity with pCO2 enrichment, leaf growth rates in C. serrulata did not increase, while those in H. uninervis and T. hemprichii significantly increased with increasing pCO2 levels. While seagrasses can be carbon-limited and productivity can respond positively to CO2 enrichment, varying carbon allocation strategies amongst species suggest differential growth response between species. Thus, future increase in seawater CO2 concentration may lead to an overall increase in seagrass biomass and productivity, as well as community changes in seagrass meadows.

Macroinvertebrate abundance in tropical Andean streams, Ecuador

Deforestation in the tropical Andes is affecting ecological conditions of streams, and determination of how much forest should be retained is a pressing task for conservation, restoration and management strategies. We calculated and analyzed eight benthic metrics (structural, compositional and water quality indices) and a physical-chemical composite index with gradients of vegetation cover to assess the effects of deforestation on macroinvertebrate communities and water quality of 23 streams in southern Ecuadorian Andes. Using a geographical information system (GIS), we quantified vegetation cover at three spatial scales: the entire catchment, the riparian buffer of 30 m width extending the entire stream length, and the local scale defined for a stream reach of 100 m in length and similar buffer width. Macroinvertebrate and water quality metrics had the strongest relationships with vegetation cover at catchment and riparian scales, while vegetation cover did not show any association with the macroinvertebrate metrics at local scale. At catchment scale, the water quality metrics indicate that ecological condition of Andean streams is good when vegetation cover is over 70%. Further, macroinvertebrate community assemblages were more diverse and related in catchments largely covered by native vegetation (>70%). Overall, our results suggest that retaining an important quantity of native vegetation cover within the catchments and a linkage between headwater and riparian forests help to maintain and improve stream biodiversity and water quality in Andean streams affected by deforestation. Also, this research proposes that a strong regulation focused to the management of riparian buffers can be successful when decision making is addressed to conservation/restoration of Andean catchments.

Bird predation experiments in tropical agroforestry landscapes of the Napu Valley in Central Sulawesi, Indonesia

We performed bird predation experiments (dummy experiments), using artificial prey and bird community data to investigate the importance of predator diversity vs. predator identity in cacao agroforestry landscapes. All sample sites were situated at the northern tip of Napu Valley in Central Sulawesi, Indonesia. After an initial mapping of the study area, we selected 15 smallholder cacao plantations as sites for our exclosure experiments in March 2010. For our predation experiment, we selected 10 (out of 15) study sites and 5 cacao trees per site for the application of artificial prey for birds (dummy caterpillars made of plasticine). Our study trees (numbered from 1 to 5 per site) were randomly chosen and we kept spacing of at least two unmanipulated cacao trees between two study trees to avoid clumped distribution. To quantify both daytime/diurnal predation and night-time/nocturnal predation (e.g. birds vs. bats), we applied 7 caterpillar dummies on all study trees and controlled them for predation marks in the early morning (05:00-06:00 am), in the evening (17:00-18:00 pm) and in the early morning on the next day (completing one survey round). In total, we performed four survey rounds per study site (in June and July 2011). The caterpillar dummies were always applied in the same order and on three different parts of each cacao study tree: One 'control dummy' (located on first branching of the cacao tree); 3 'branch dummies' (located on one main branch coming from first branching; 20-25 cm between single dummies) and 3 'leaf dummies' (3 medium aged cacao trees adjacent to main branch were selected and single dummies placed in the center of each cacao leaf). The different positions were chosen to control for different foraging modes of predators (e.g. branch gleaners versus leaf gleaners). During day- and nighttime surveys, we controlled if the dummy caterpillars were still present in their original position, if they were absent and could not be relocated on the ground or if they were fallen to the ground, but could still be recorded. Eaten dummies were counted as 1 mark usually, except for those dummies, where two or more different kind of arthropods had eaten parts of the dummy (2 marks or more). Other predation marks were added to this number. For each dummy, we counted the total number of different predation marks. We focused on predation marks that could be identified with certainty (based on preliminary observations and/or literature): marks of birds, rodents and snails. Finally, we analysed the relationship of bird predation marks and bird community parameters (abundance vs. diversity), as well as effects of local and landscape management on the avian predation success.