Moderate Primary Productivity Research Articles

Soil organic matter dynamics in tiger bush (Niamey, Niger). Preliminary results

Some typical features of soil organic matter dynamics and soil texture were studied to discuss the particular spatial pattern of tiger bush in Niger and its dynamics. The soil texture through silt and clay contents showed a high variability in the vegetation arc as well as in the bare area. These variations were clearly linked to water/wind erosion and termite activity. Tiger bush soils showed a high capacity to store soil organic matter despite a moderate primary production, even in the bare area where the input of plant debris has been nil for many decades. The carbon content was higher within the vegetation arc (0.93 %) than within the bare area (0.45 %). Additionally, potential carbon mineralisation significantly varied in relation to the total carbon content and thus to primary production. Then, the vegetation arcs can be viewed as `fertility islands' as in many arid ecosystems. The measurements of δ13C showed a dominant contribution of C3 plants to the soil organic matter pool. Nevertheless, the contribution of C4 plants was not negligible. Two hypotheses could be proposed: a different mineralisation rate between C3 and C4 plants; or (ii) a better physical protection of C4 compounds against biodegradation. The soil variables depending totally or partly on biological factors, such as carbon and nitrogen contents, carbon isotopic composition, carbon potential mineralisation, did not show any symmetry in their variations along the studied transects. It was expected in the vegetation arc because the vegetation cover does not show symmetry in its specific composition and spatial structure. In the bare area, a clear asymmetry was observed on some of the variables: carbon content, fine material content and natural abundance of 13C. This supports the hypothesis that the vegetation arcs move upslope, and weakens the hypothesis of the alternance of contraction and spreading periods of the vegetation cover.

ON THE VERTICAL DISTRIBUTION OF LIVING (ROSE BENGAL STAINED) BENTHIC FORAMINIFERS IN THE ARCTIC OCEAN

The vertical distribution of living (Rose Bengal stained) benthic foraminifers was determined in the upper 15 cm of sediment cores taken along transects extending from the continental shelf of Spitsbergen through the Eurasian Basin of the Arctic Ocean. Cores taken by a multiple corer were raised from 50 stations with water depths between 94 and 4427 m, from areas with moderate primary production values to areas that are among the least productive ones in the world. We believe, that in the Arctic Ocean the vertical distribution of living foraminifers is determined by the restricted availability of food. Live foraminiferal faunas are dominated by potentially infaunal species or epifaunal species. Species confined to the infaunal microhabitat are absent in Arctic sediments that we examined, and predominantly infaunal living species are nowhere dominant. In general, an infaunal mode of life is restricted to the seasonally ice-free areas and thus to areas with at least moderate primary production during the summer period. Under the permanent ice cover living species are usually restricted to the top centimeter of the sediment surface, even though some are able to dwell deeper in the sediment under ice-free conditions.

Lack of organic matter accumulation on the upwelling-influenced Somalia margin in a glacial-interglacial transition

Monsoon-induced upwelling has been operating off Somalia for at least the last 160,000 years, making the NW Indian Ocean one of the most productive oceanic areas in the world. We have studied the sediments (recovered during the Indusom cruise of the R/V Marion-Dufresne) from three different environments off this margin; a core from the highly productive surface waters (core MD85-674); a core where the oxygen minimum intersects the slope (core MD85-664); and a reference core from the deep basin with normal bottom oxygen levels and moderate primary production (core MD85-668). Though the first two environments are a priori favourable to organic matter (OM) accumulation, no marine OM enrichment can be observed in the cores. To understand this anomaly, emphasis is placed upon the transition from isotopic Stage 6 to Stage 5, because the end of the glacial Stage 6 is characterised by intensified upwelling and primary productivity in this region, whereas Stage 5 experienced a marked decrease in productivity. Various palaeoenvironmental markers, such as OM, V, Ni, Cu, Zn, Ba and P, have been used to identify periods of high productivity and/or bottom water oxygen-depletion. The results suggest that (1) the lack of marine OM in two settings favourable to organic carbon storage may be due to the nature of primary productivity. Coccolithophorid production would not be prone to OM accumulation, in contrast to the contribution of OM by diatoms or, particularly, naked phytoplankton (dinoflagellates or other peridinians); (2) Ba is not an accurate palaeoproductivity marker in this region although the oceanic environment would have been a priori suitable for the use of Ba as a proxy indicator; (3) the period of intensified upwelling in Stage 6 as well as the Stage 6-5 transition left no geochemical imprint on the sediments. This means that some upwelling systems may not be accompanied by marked enrichments in marine organic matter within the underlying sediments. In other words, the Somalia upwelling does not participate to the so-called biological pump and has no positive effect upon long-term carbon storage.

Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean

THE iron hypothesis1–3—the suggestion that iron is a limiting nutrient for plankton productivity and consequent CO2 drawdown— has been tested by small-scale experiments in incubation bottles in the subarctic Pacific2,4 and Southern5 –7 Oceans, and by a recent large-scale experiment in the equatorial Pacific Ocean8,9. Here we test the idea by looking at natural levels of productivity in regions of the Southern Ocean with differing iron abundance. In the southerly branch of the Antarctic circumpolar current (ACC), upwelling of deep waters supplies sufficient iron to the surface to sustain moderate primary production but not to permit blooms to develop. In contrast, within the fast-flowing, iron-rich jet of the polar front (PF), spring blooms produced phytoplankton biomass an order of magnitude greater than that in southern ACC waters, leading to CO2 undersaturation. The plankton-rich PF waters were sharply delineated from adjacent iron-poor waters, indicating that iron availability was the critical factor in allowing blooms to occur.

Gross and net primary production in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988

Daily rates of gross and net primary production were calculated in the Scotia-Weddell Sea sector of the Southern Ocean during spring 1988 (EPOS, Leg 2) on the basis of kinetic experiments, which combine radiotracer technology and classic biochemical procedures, and by taking into account the light regime, the physical structure of the water column, the vertical distribution of chlorophyll a, and the protozoan grazing pressure. From these calculations, three distinct sub-areas were identified: the Closed Pack Ice Zone (CPIZ), characterized by the lowest average gross primary production (0.36 gC · m−2 · day−1); the Marginal Ice Zone (MIZ) with a maximum mean value of 1.76 gC · m−2 · day−1; and the Open Ocean Zone off the ice edge (OOZ) with an intermediate mean value of 0.87 gC · m−2 · day−1. Net primary production fluctuated nearly in the same proportions, averaging 0.55, 0.2 and 1.13 gC · m−2 · day−1 in the OOZ, CPIZ and MIZ respectively, representing 53% of the total photo-assimilated carbon under heavy ice cover (CPIZ) and 64% in the two other areas. Available light, strongly dependent on the ice cover, was shown to control the level of primary production in the sea ice associated sub-areas, whilst protozoa grazing on phytoplankton determined the moderate primary production level characteristic of the “well illuminated” OOZ area.

Interspecific Interactions Among Phytophagous Insects of Tallgrass Prairie: An Experimental Test

Densities of the grasshopper Phoetaliotes nerbrascensis (Acrididae) were manipulated in native tall grass prairie at Konza Prairie, Kansas, to measure competitive and facilitative effects on densities of co—occuring phytophagous insects. In late June 1987, P. nebrascensis nymphs were transferred among enclosed plots (each 550—900 m2) to produce three pairs of plots with grasshoppers either removed or added. Pretransfer densities in the six plots were estimated at °5 nymphs/m2, 2—2.5 nymphs/m2 were then removed or added. Densities of the manipulated species and other abundant phytophagous insects were subsequently monitored (by sweep net) through mid—August. The manipulations in late June resulted in mean densities of P. nebrascensis nymphs 1.5–4 times as large on addition plots as on removal plots; these differences persisted through mid—August. No significant differences between treatments were found, however, in the numbers of individuals of other Acrididae or Tettigoniidae on four post—transfer sampling dates (July to mid—August). Significant differences were also not found in the numbers of individuals of Phasmatidae, Homoptera, or phytophagous Hemiptera and Coleoptera, or in the biomasses of live grass, dead grass, or forbs present in mid—August. 1987 was year of moderate primary production on Konza Prairie. The manipulations produced experimental treatments that mimicked intermediate vs. relatively high natural densities of P. nebrascensis. The results indicate that under these conditions, changes in P. nebrascensis density have little net short—term impact, competitive or otherwise, on the population dynamics of co—occurring grasshoppers or other phytophagous insects.