Changes In Ocean Productivity Research Articles

Primary productivity and the carrying capacity for herring in NE Pacific marine ecosystems

The carrying capacity for Pacific herring ( Clupea pallasi) and its relationship to primary productivity was examined for eight populations in the NE Pacific and eastern Bering Sea. Data on biomass (ages 3 and older) and catches of herring in British Columbia (Georgia Strait, west coast Vancouver Island, Central Coast, Queen Charlotte Islands, Prince Rupert) and Alaska (Sitka, Prince William Sound, eastern Bering Sea) during the second half of the 20th Century derived from analytical stock assessments were used to calculate annual surplus production of herring. There was considerable interannual variability in herring productivity among all populations, however, only three (Georgia Strait, Prince Rupert, Prince William Sound) showed significant differences in mean productivity on decadal time scales (productivity regimes). Carrying capacity for the most recent productivity regime for each population was estimated using the Schaefer surplus production model. Mean annual primary productivity was estimated from remotely-sensed (SeaWiFS) chlorophyll data for British Columbia and Sitka ecosystems, and from in situ chlorophyll data for Prince William Sound and the eastern Bering Sea. The carrying capacity for herring populations in the NE Pacific ranged from 28,000 to 250,000 tonnes, and to 325,000 tonnes in the eastern Bering Sea. When considered on the basis of their distributional area, the west coast of Vancouver Island and Georgia Strait populations had the highest carrying capacity per unit area (9.3–13.8 tonnes km −2) and the eastern Bering Sea had the lowest (0.7 tonnes km −2). There is a significant positive linear relationship between the productivity of herring populations at carrying capacity and primary productivity on a per unit area basis. Although similar direct relationships have been observed between phytoplankton standing stock (as chlorophyll biomass) and total catches of resident fish populations from these regions, such a direct relationship was unexpected between primary productivity and herring carrying capacity because of multiple trophic pathways and other factors (e.g. spawning habitat) which should limit the carrying capacity for individual populations of particular species of fish. These results may be used to forecast potential responses of herring in NE Pacific ecosystems to global changes in ocean productivity.

Ecosystem models show combined effects of fishing, predation, competition, and ocean productivity on Steller sea lions (Eumetopias jubatus) in Alaska

Steller sea lions (Eumetopias jubatus) increased in the eastern portion of their range while declining in the Gulf of Alaska and Aleutian Islands from the late 1970s to late 1990s. We constructed ecosystem models of the central and western Aleutians and of southeast Alaska to simultaneously evaluate four hypotheses explaining sea lion dynamics: killer whale (Orcinus orca) predation, ocean productivity, fisheries, and competition with other species. Comparisons of model predictions with historical time series data indicate that all four factors likely contributed to the trends observed in sea lion numbers in both ecosystems. Changes in ocean productivity conveyed by the Pacific Decadal Oscillation influenced the abundance trajectory of several species. Fishing could have affected the ecosystem structure by influencing the abundance of Atka mackerel (Pleurogrammus monopterygius) in the Aleutians and Pacific herring (Clupea pallasii) in southeast Alaska. Pacific halibut (Hippoglossus stenolepis) in the Aleutians and arrowtooth flounder (Atheresthes stomias) in southeast Alaska appear to impede sea lion population growth through competitive interactions. Predation by killer whales was important when sea lions were less abundant in the 1990s in the Aleutians and in the 1960s in Southeast Alaska, but appear to have little effect when sea lion numbers were high.

Editorial: Late Holocene oceanographic and climate change from the western European margin: the results of the HOLSMEER project

The underlying aim of the HOLSMEER project has been to improve our understanding of natural climate variability through the search for, interpretation and quantification of, climatic variability in very high-resolution shallow marine records from Atlantic Europe covering the last 2000 years. This has been achieved through detailed analyses of a series of coastal and shallow marine sites spanning the Atlantic seaboard from Iberia to western Norway, and extending across to Iceland. HOLSMEER partners have documented pronounced instability in the thermohaline circulation (THC) during the period immediately prior to the recent significant anthropogenic impact on the environment. For the first time we have been able to document that these changes in the coastal ocean are correlated with significant changes in terrestrial palaeoclimate proxies, notably during the last 1000 years. The notable changes are the significance of warm sea surface temperatures (SST) associated with active THC between AD 700 and 1000, a transition phase to much colder SST and reduced THC between AD 1000 and AD 1300, colder SST through to AD 1900 followed by an active re-establishment of warm surface water circulation during the twentieth century. These switch-like reorganizations of the climate system have influenced the entire seaboard from western Iberia to western Norway, and have forced changes in ocean productivity, iceberg frequency and sea ice coverage. These changes have also directly influenced sea level through steric effects. The project has also resulted in significant advances in the establishment of new palaeoclimate proxies, including transfer functions related to benthic foraminifera, diatoms and dinoflagellate cysts. Annual growth band series from fossil specimens of the long-lived bivalve mollusc Arctica islandica from the northern North Sea have been successfully cross-matched, and independently verified by radiocarbon dating, to provide the longest Arctica chronology, and the first floating chronology constructed entirely from marine fossils.

Reference points for coho salmon (Oncorhynchus kisutch) harvest rates and escapement goals based on freshwater production

We describe a simple scheme for the management of coho salmon (Oncorhynchus kisutch) population aggregates that uses reference points derived from an empirical analysis of freshwater production data. We fit a rectilinear "hockey stick" model to 14 historical data sets of female spawner abundance and resulting smolt production and found that at low spawner abundance, the average productivity was about 85 smolts per female spawner. Variation in productivity among streams may be related to the quality of the stream habitat. We show how freshwater productivity can be combined with forecasts of marine survival to provide a limit reference point harvest rate. Our method will permit harvest rates to track changes in ocean productivity. We also used the historical data to estimate that, on average, a density of 19 female spawners·km-1 is required to fully seed freshwater habitats with juveniles. However, there was considerable variation among the streams that might limit the utility of this measure as a reference point. Uncertainty in the forecasts of marine survival and other parameters needs to be incorporated into our scheme before it can be considered a precautionary approach.

Role of sea‐surface temperature and ocean circulation changes in the reorganization of the global carbon cycle at the Last Glacial Termination

The increase in atmospheric CO2 mixing ratio at the close of the Pleistocene epoch was part of a larger reorganization of the global carbon cycle that involved the regrowth of the terrestrial biosphere and soil reservoirs, changes in the circulation and surface temperature of the oceans, and possibly changes in oceanic productivity and the throughput of oceanic alkalinity. Here we use a box model of the global carbon cycle to highlight and investigate the role that two of these mechanisms played in distinguishing the carbon dioxide and carbon isotope budgets of glacial versus interglacial conditions: the increase in ocean surface temperature and the circulation‐driven change in export productivity as the last glacial period came to a close. The first of these is shown to explain the otherwise paradoxical isotopic lightening of atmospheric CO2 despite the enhanced sequestration of organic carbon in the deep ocean under glacial conditions. The second provides a new mechanism for increasing whole‐ocean carbonate ion activity, with the expected effect of chemically decreasing CO2 partial pressure in the surface ocean. It is argued that the implied reorganization of the oceanic carbon budget is consistent with both the steady state and the transient features of deglacial carbon cycle history, as well as with the timescale for the atmospheric CO2 increase at glacial termination.

Signature of Non-steady-state Diagenesis in Continental Shelf Sediments

The phenomenon of non-steady-state diagenesis is well recognized in open ocean sediments and occurs either due to the deposition of turbidites on pelagic sediments or to changes in ocean productivity. However, the occurrence of oxidized sediment below reducing sediments in many near-shore and continental shelves indicates that non-steady-state diagenesis takes place even in shallow-water regimes. This paper shows that oxic conditions on the continental shelf result from low sedimentation rates induced by the rapid rise in sea level during the early Holocene to the beginning of late Holocene time, whereas a comparatively high rate of sedimentation (with increased burial of organic matter, generating reducing conditions) due to a relatively stable sea level during the late Holocene, is responsible for the initiation of non-steady-state diagenesis. The lower oxidized sediments have not yet experienced a sufficient flux of reductants to mobilize base metals. In contrast, base metals in the upper reducing sediments have undergone mobilization and diffused out of the sedimentary column to the overlying water.

Cenozoic deep-sea benthic foraminifers: Tracers for changes in oceanic productivity?

From late middle Eocene through earliest Oligocene, high-latitude regions cooled, and by the end of the period, continental ice sheets existed in Antarctica. Diversity of planktonic microorganisms declined, and modern groups of terrestrial vertebrates originated. Coeval faunal changes in deep-sea benthic foraminifers have been related to cooling of deep waters and increased oxygenation. Cooling, however, occurred globally, whereas species richness declined at high latitudes and not in the tropics. The late Eocene and younger lower-diversity, high-latitude faunas typically contain common Epistominella exigua and Alabaminella weddellensis , opportunistic phytodetritus-exploiting species that indicate a seasonally fluctuating input of organic matter to the sea floor. We speculate that the species-richness gradient and increase in abundance of phytodetritus-exploiting species resulted largely from the onset of a more unpredictable and seasonally fluctuating food supply, especially at high latitudes.

231Pa/230Th ratios in sediments as a proxy for past changes in Southern Ocean productivity

THE biological productivity of the oceans is sensitive to changes in climate, which can affect essential factors such as nutrient and light availability. In turn, ocean productivity may influence climate by regulating the partitioning of carbon dioxide, a greenhouse gas, between the ocean and the atmosphere. Investigators have attempted to link variations in atmospheric CO2 content, recorded in ice cores1,2, to the productivity of the Southern Ocean3–6, but an unambiguous means of assessing past changes in ocean productivity has been lacking. Here we exploit established relationships between 231Pa/230Th ratios and particle flux7–12 to infer, from the analysis of dated sediment cores, variability through time of fluxes of particulate biogenic material exported from surface waters. Records from two cores in the Atlantic sector of the Southern Ocean indicate that ocean productivity during glacial periods was lower than at present south of the Antarctic polar front, and support earlier conclusions13–16 that the zone of maximum productivity migrated northwards during glacial conditions. Although further work at other sites is needed for an assessment of changes in total Antarctic productivity, our technique has the potential to provide this information while avoiding some of the limitations of other productivity proxies.

Reassessment of Crete (Greenland) ice core acidity/volcanism link to climate change

Previous comparisons of the Crête (Greenland) ice core acidity record with climate variations of the last 1400 years suggested that changes in volcanism may have significantly influenced climate on this time scale. However, the ice core acidity record consists of volcanically‐induced spikes superimposed on a background acidity of nonvolcanic origin that varied significantly in amplitude. Herein we produce a record of Crête volcanism based solely on an objective definition of individual volcanic events in the ice core record. There are 92 acidity peaks in the 1420‐year record. Since at least 26% of the post‐1600 eruptions are of local (Icelandic) origin, we estimate a mean recurrence interval of 20.8 years for hemispheric‐scale eruptions rich in sulphate. On decadal time scales, volcanism has varied by a factor of three‐four over the last 1400 years. However, the climate‐volcanism correlation is considerably less impressive (r = −0.23) than previously concluded (r = −0.52). The good correspondence between volcanism and climate previously reported are due to background acidity levels that show a significant Little Ice Age increase. The background increase may reflect changes in ocean productivity.

Climatic and oceanographic isotopic signals from the carbonate rock record and their preservation

AbstractStable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation. Oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases. Terrestrial carbonates–meteoric cements, calcretes and speleothems–similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover.Primary environmental isotopic signals may be obscured by the effects of post-depositional diagenetic alteration. Cementation and replacement reactions can take place in a wide range of diagenetic environments; the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting, together with subsequent changes in relative sea-level and burial history. Carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated. Linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration: water-rock interaction and fluid mixing may produce non-linear distributions.Attempts to determine long-term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow-water limestones must include an assessment of the diagenetic history of the materials analysed. Pétrographic examination using conventional microscopy backed up, where appropriate, by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration. Where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary, environmental, isotopic signals. Recent research has shown that these techniques can be successfully employed in both Phanerozoic and Precambrian sediments.

A paleoceanographic model for the early evolutionary history of planktonic foraminifera

The first major radiations of planktonic foraminifera occurred during the mid-Cretaceous. However, this period of great diversification was interrupted by two intervals of pronounced paleoceanographic/paleoecologic change, one during the latest Aptian-early Albian and a second associated with the Cenomanian-Turonian boudary. Both of these times corresponded with widespread organic carbon burial during intervals commonly referred to as Oceanic Anoxic Events (OAE) 1 and 2, respectively. This synthesis focuses on the evolutionary history of Aptian-early Cenomanian planktonic foraminifera and the relationship with OAE 1. Dynamic paleoceanographic changes controlled the course of planktonic foraminiferal evolution during the mid-Cretaceous. Heightened plate tectonic activity, a major long-term rise of global sea level and an overall increase of global temperatures characterized Aptian-Albian time. As proposed here, the evolutionary history of Aptian-early Cenomanian planktonic foraminifera was influenced most strongly by changes in the density structure and stability of the upper water column via changes in the sites and rates of water mass production through time, and perhaps by subtle changes in oceanic productivity (including trophic structure) and shifts in productivity centers with rising sea level. The creation and growth of epicontinental seas played a major role in regional climate change, water mass production and water column structure. Epicontinental seas were also an important factor in creating vast new niche space and increased biotic diversity for many marine organisms. The amplified seasonality and instability of epicontinental seas resulted in habitation of the upper water column by morphologically simple, eurytopic planktonic foraminifera. Several new species of epicontinental sea dwelling planktonic foraminifera appeared during the mid-Cretaceous. However, much of the diversification of morphologic form took place in oceanic areas during times of oceanic stability and upper water column stratification during mid-to late Aptian and late Albian time. A decline in diversity and return to simple morphotypes occurred when oceanic stability was disrupted and the upper water column was poorly stratified during latest Aptian-early Albian time.

Productivity change as a control on planktonic foraminiferal evolution after the cretaceous/tertiary boundary

The rate of evolution of Palaeocene and Early Eocene planktonic Foraminifera is examined using taxonomic data from two distant sites. The two million years immediately following the Cretaceous/Tertiary boundary was characterised by low rates of evolution within the group. Peaks in the rate of species turnover at 62.5 and 57.5 Ma suggest a close relationship between the evolution of this group and changes in ocean productivity and therefore the intensity of the sub‐surface oxygen minimum zone. A model for the evolution of the early Palaeogene planktonic Foraminifera is proposed.

Danian faunal succession: Planktonic foraminiferal response to a changing marine environment

Planktonic foraminiferal evolution following the Cretaceous/Tertiary boundary extinction event is marked by a series of events in which one assemblage is rapidly replaced by another. This pattern is characteristic of a highly changing pelagic environment. The observed changes in the planktonic foraminifera may be related to long-term changes in oceanic productivity that persisted for about the first 2 m.y. of the Danian.