Trends In Primary Production Research Articles

Comparison of global net primary production trends obtained from satellite‐based normalized difference vegetation index and carbon cycle model

The global terrestrial net primary production (NPP) trend was estimated from two independent methods, satellite observation data and a carbon cycle model, and the results were compared for validation. The satellite‐based NPP trend was estimated from the incoming surface solar radiation data set and a National Oceanic and Atmospheric Administration/ Advanced Very High Resolution Radiometer data set that was corrected by normalized difference vegetation index in areas of desert and dense vegetation. The increase in NPP from the Goddard Institute for Space Studies solar radiation data set and from the LaRC solar radiation data set over 10 years in the 1980s was estimated to be 1.8 and 4.4%, respectively. The NPP trend based on a carbon cycle model was estimated from a simple carbon cycle model that was established for the period 1850–1990 with biospheric and oceanic carbon cycle history constraints. The historical trend obtained from the model correlates well with the time variation of not only the observed atmospheric CO2 but also the biospheric and oceanic carbon cycle history. Terrestrial NPP shows an increasing trend beginning in 1930 and is estimated to increase at a rate of 1.1% over the 10‐year period in the 1980s. Although all these methods show a recent increase in NPP, satellite‐based estimation using the LaRC data set shows a larger trend than the others. A comparison of he trends estimated by these methods indicates that it is necessary to improve the accuracy of incoming surface radiation data, CO2 emission history from changes in land‐use change and model structure.

Changes in the trophic state of Lake Erie: discordance between molecular [formula omitted] and bulk [formula omitted] sedimentary records

Prior to 1980, anthropogenic phosporus (P) loadings resulted in increased algal productivity and the deterioration of water quality within Lake Erie. We measured C and N elemental compositions, geolipid concentrations, and δ 13 C values for bulk organic material and individual alkanes in samples taken from a 40-cm sediment core from the eastern basin of Lake Erie to infer recent trends in primary productivity and diagenesis. The predominance of autochthonous algal-derived C throughout the core is evidenced by low C/N values (9–10) and the persistent abundance of n-C 15 to n-C 17 alkanes throughout the core. The δ 13 C values of bulk organic C from Lake Erie vary between −25.9 and −24.9‰ with a maximum corresponding to the period of elevated historical P loading. Within Lake Erie, δ 13 C values of short-chained algal-derived alkanes (< n-C 18) are lower (approximately 1.0‰) than their terrestrial-derived homologs (> n-C 25). Down-core variations in the δ 13 C composition of algal-derived short-chained alkanes do not exhibit the same trend as that observed for bulk organic C. Down-core variation in δ 13 C values of individual alkanes likely reflect multiple diagenetic effects including kinetic fractionation effects, variations in the relative contributions of isotopically unique n-alkanes to the sediments, and/or temporal changes in 13 C enrichments of n-alkanes associated with a single source. High sedimentation rates characteristic of this core emphasize temporal variations in molecular δ 13 C values that might not otherwise be observed. Whereas δ 13 C data for bulk sedimentary organic C appear to reflect variation in trophic state, the isotopic record for algal-derived n-alkanes (e.g., n-C 17 and pristane) from Lake Erie must be considered from the perspective of molecular transformations.

Primary production and phytoplankton in Lake Kinneret: A long‐term record (1972‐1993)

The long‐term (22 yr) record of primary production in Lake Kinneret, Israel, has been examined, together with chlorophyll and microscopically determined algal biomass, in order to discern whether there have been any significant changes in these parameters during this period. During the period 1972 through 1993, annual averages have ranged from 1,223 to 2,311 mg C m−2 d−1 for primary production, from 127 to 246 mg m−2 for chlorophyll, and from 39.3 to 98.5 g m−2 for algal wet weight. The annual peak of these parameters was in April–May. Over 22 yr, variability in primary productivity was more closely related to changes in chlorophyll than to changes in algal biomass. No evidence was found for consistently increasing long‐term trends in primary production, chlorophyll concentrations, or algal wet weight biomass from 1972–1993. Although the annual and semiannual averages of algal biomass were significantly higher in the past 11 yr than those in the previous decade, this pattern could arise from a long‐term cyclical but self‐compensating trend. The extended record indicates that despite population growth and intense economic development around the lake and in its catchment area, there has been no extreme eutrophication of Lake Kinneret from 1972 to 1993. We suggest that this relative resiliency of the Kinneret ecosystem is due to high ambient levels of alkalinity, calcium, and pH in the lake water acting to limit phosphorus availability, which in turn restricts the outgrowth of phytoplankton.

Seasonal distribution of nutrients and primary productivity on the eastern continental shelf of Venezuela as influenced by the Orinoco River

Nitrogenous nutrients, dissolved silicate, and salinity were measured in surface waters and shallow hydrocasts along similar cruise tracks during the spring (dry season) and fall (wet season) of 1988. Both cruises transected the eastern Caribbean, transited the Gulf of Paria, ran parallel to the Orinoco Delta and into the main channel of the Orinoco River. Trends in primary productivity were also measured by daily carbon 14 incubations. In both seasons, samples covered the range from highly oligotrophic and transparent to highly productive and rich in biogenic and abiogenic particulate matter. Most of the Orinoco outflow appears to turn N to NW and remains in shallow waters off Venezuela and surrounding Trinidad, permitting benthic regeneration of river‐borne nutrients. However, the role of the Orinoco and associated low‐salinity coastal waters in fertilizing large areas of the eastern Caribbean basin, as suggested by satellite imagery, can be approximated crudely from the nutrient composition at Boca de Dragon, which is representative of the nutrient status of these waters as they flow into deeper Caribbean waters. Additional nutrients may be supplied to the area primarily from Amazon‐derived water entering the Caribbean Basin further north, with some coastal upwelling along the continental shelf in the dry season.

Activity of the respiratory electron transport system and respiration rates within the oxygen minimum layer of the Arabian Sea

Abstract Measurements of the activity of the respiratory electron transport system (ETS) at 15 stations in the Arabian Sea during the northeast monsoon (December 1988) yield high respiration rates that do not correlate with the trends in primary productivity. The rates are unlikely to be sustained by the supply of carbon associated with the sinking particles alone, and seem to suggest a major role for dissolved and/or suspended organic matter in fuelling oxygen consumption and denitrification. The data are utilized to compute a denitrification rate of 24–33 Tg N y−1 in the Arabian Sea. This estimate agrees with the estimate based on the exports of nitrate deficits outside the denitrification zone. The ventilation time of the denitrifying layer is calculated as ∼1 year.

Recent changes in productivity and climate of Lake Ontario detected by isotopic analysis of sediments

Cores from Lake Ontario provide a record of lake responses to increased phosphorus loading that began after early forest clearance in the mid‐1800s. We measured 13C/12C ratios of organic and inorganic C (calcite) in the sediments to infer recent trends in primary production. The δ13C of organic C increased in response to historic increases in P enrichment until the mid‐1970s and then decreased in response to reduced P loading. The δ13C of calcite increased after 1940 but did not decrease in response to reduced P loading. We conclude that the results differ because δ13C of organic C is determined by primary production early in the seasonal cycle when flux of organic C to the sediments is greatest, whereas δ13C of calcite depends on the isotopic composition of dissolved inorganic C during late summer when calcite is precipitated.Calcite accumulation and its δ13C ratio oscillated after 1940 with peaks estimated at 1948, 1966, 1977, and 1983. These peaks correspond to minima in δ18O values, indicating calcite precipitation at higher temperature, and to years of early thermal stratification. We hypothesize that calcite accumulation and its stable isotopic ratios are controlled by interannual variability in onset of thermal stratification of the lake. Our results thus suggest a direct link from climate (i.e. thermal stratification) to interannual variation in lake productivity and to carbonate precipitation and its δ13C signature.

Light‐Productivity Model for Onondaga Lake, N.Y.

Two models were calibrated to the 1978 primary productivity conditions of nutrient‐rich hypereutrophic Onondaga Lake, N.Y.; one was based on the approach of Bannister, the other on a more conventional format. The data base, necessary for model development, included continuously monitored incident photosynthetically active radiation (PAR), and 115 parallel measurements of water column temperature, PAR attenuation, chlorophyll a, primary productivity, and key nutrients. Values of key parameters for the Bannister model, obtained by evaluation of the data set, differed from those recommended in the literature. Both models effectively predicted seasonal and shortterm trends in primary productivity for most of the study, though the Bannister model was slightly better. The Bannister model predictions were particularly sensitive to estimates of the maximum quantum yield of photosynthesis. The Bannister model is preferred on theoretical grounds, since it incorporates more fundamental and more constant parameters, ...

Australia: Trends in primary production

Australia: Trends in primary production