John A. McGowan

Climatic Warming and the Decline of Zooplankton in the California Current

Since 1951, the biomass of macrozooplankton in waters off southern California has decreased by 80 percent. During the same period, the surface layer warmed—by more than 1.5�C in some places—and the temperature difference across the thermocline increased. Increased stratification resulted in less lifting of the thermocline by wind-driven upwelling. A shallower source of upwelled waters provided less inorganic nutrient for new biological production and hence supported a smaller zooplankton population. Continued warming could lead to further decline of zooplankton.

Climate and Chlorophyll a: Long-Term Trends in the Central North Pacific Ocean

Since 1968 a significant increase in total chlorophyll a in the water column during the summer in the central North Pacific Ocean has been observed. A concomitant increase in winter winds and a decrease in sea surface temperature suggest that long-period fluctuations in atmospheric characteristics have changed the carrying capacity of the central Pacific epipelagic ecosystem.

The biological response to the 1977 regime shift in the California Current

Among the least understood interactions between physics and biology in the oceans are those that take place on the decadal scale. But this temporal scale is important because some of the greatest ecological events take place on this time scale. More than 50 years of measurement in the California Current System have revealed significant ecosystem changes, including a large, decadal decline in zooplankton biomass, along with a rise in upper-ocean temperature. The temperature change was a relatively abrupt shift around 1976–77, concurrent with other basin-wide changes associated with an intensification of the Aleutian Low-pressure system. This intensification generates temperature anomalies in the ocean by altering the patterns of net surface-heat fluxes, turbulent mixing, and horizontal transport. Changes in the mean abundance of zooplankton in the southern California Current have been attributed to variations in the strength of coastal upwelling, variations in the horizontal transport of nutrient-rich water from the north, or increased stratification due to warming, all of which could be affected by fluctuations in the Aleutian Low. Here we show that a deepening of the thermocline accompanied the warming and increased the stratification of the water column, leading to a decrease in the supply of plant nutrients to the upper layers. This is the most likely mechanism for the observed plankton decline, and subsequent ecosystem changes. A global change in upper-ocean heat content, accompanied by an increase in stratification and mixed-layer deepening relative to the critical depth for net production, could lead to a widespread decline in plankton abundance.

Structure in the Copepod Community of the North Pacific Central Gyre

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Dominance and Diversity Maintenance in an Oceanic Ecosystem

Disturbance—perturbation, dispersal—reaction, and contemporaneous disequilibrium are similar theories used to explain the maintenance of species diversity in communities. These theories explicitly predict that in patches, on certain time—space scales, there should be substantial shifts in the order of species dominance. There is good evidence that these theories may explain species coexistence in terrestrial and marine systems of sessile organisms. We have tested this set of theories in a mobile pelagic system by examining the order of dominance of copepod species in samples separated in time and space, collected from °30 min to 16 yr apart, and from hundreds of metres to thousands of kilometres apart. We could not detect significant changes among the samples in rank order or in percent similarity of species abundance on any time scale, or on any space scale up to °800 km, either when all 175 species or when only the 30 most abundant were considered. There was small—scale, mesoscale, seasonal, and interannual heterogeneity in physical properties during the time we made our measurements. Although the theories are satisfactory explanations of diversity maintenance in sessile systems, our results fail to validate them in our mobile pelagic system. Because there were episodes of significant physical variability and because of the long—term species equilibrium and constancy of dominance, we believe our highly diverse community to be resilient and robust, rather than fragile. The regulatory forces are strong and almost certainly biological, rather than physical, but we cannot identify them.

Mixing and oceanic productivity

Abstract An unusual oceanographic event in 1969 has allowed us to investigate how nutrients are mixed into the euphotic zone of the oligotrophic central gyre of the North Pacific. A doubling of the rate of primary production and a significant increase in the standing crop of zooplankton were the original evidence. Our interpretation of these and of the physical data has led us to believe that a mixing episode, or rather a series of small mixing events, took place. We hypothesize that the mixing was upward from below the euphotic zone rather than downward from the surface. We observed a layer where the depth variance of isotherms was at a maximum. This was also a maximum in the frequency of temperature inversions. While the layer could be due to a large number of intrusions of water of anomalous temperatures and therefore at a given density anomalous salinity, our T-S diagrams do not indicate that such anomalous salinities were present. Thus we interpret the depth variance of the isotherm layer as being the result of up and down movement of the water, perhaps internal waves. Although present in other years, this layer was shoaler in 1969, bringing it to the top of the nutricline and closer to the bottom of the euphotic zone. We suggest that shear induced turbulence or breaking internal waves, or both, may act as a nutrient pump in an otherwise stably stratified water column.

Oceanic habitat differences in The North Pacific

Abstract A pronounced faunal boundary and a gradient of Zooplankton biomass exist in the oceanic North Pacific. A summer and a winter set of observations were taken on transects normal to these latitudinal boundaries. The resulting data have been analysed both in terms of the measured variables from either side of the boundary area and of their interrelationships. Great differences between three major habitats in concentrations of nutrients, standing crops of phyto- and Zooplankton, average temperatures and average salinities have been demonstrated. Furthermore, the type of seasonal changes in these variables differed between habitats. In general, variations in Zooplankton abundance are related to plant abundance in the summer and phosphate-phosphorus in the winter. Horizontal and vertical temperature gradients are not strongly related to variations in Zooplankton abundance nor is the dissolved plus particulate organic carbon concentration; however, there is evidence that the dissolved plus particulate organic carbon does not behave as a conservative property either within or below the mixed layer. There appear to be some qualitative differences in fundamental ecological processes between these habitats.

The relationship of the distribution of the planktonic worm, Poeobius meseres Heath, to the water masses of the North Pacific☆

Abstract The distribution of a planktonic worm, Poeobius meseres Heath , was determined from an examination of over 1800 quantitative plankton tows taken in the North and South Pacific. This distribution is compared with the distribution of water masses, as defined by temperature-salinity curves. Since the water mass concept involves a three dimensional unit of the ocean, its use in describing the environment of an animal whose distribution is also three dimensional, is preferable to the method of comparing plankton distributions with horizontal isotherms or isohalines. The distribution of Poeobius coincides, for the most part, with that of the Subarctic water mass and the transition region of Subarctic water, the California Current. However, a few specimens were found in the eastern tropical Pacific. A satisfactory explanation for its restricted presence in this latter area is not possible at this time, but there is some evidence to indicate that this southern segment of the population is not endemic but has been carried in from the north. If this is true, then the occurrence of Poeobius here must be accounted for in considering the sources of the Intermediate water of the area.

The shallow salinity minimum and variance maximum in the central North Pacific

Abstract The variance maximum is a depth zone in the main pycnocline of the central North Pacific with relatively greater heterogeneity in temperature, salinity, and density. It is a depth zone characterized by an intrusion of low-salinity water (the shallow salinity minimum), numerous finescale temperature inverssions, and a maximum in the depth variance of isotherms and isopycnals. The variance maximum feature was apparent on each of five summers, but it varied between years in depth and temperature-salinity characteristics. The observation that there is both a low-salinity intrusion and relatively greater water motion (shown by the isopycnal variance maximum) is consistent with relatively enhanced mixing in the depth zone of the variance maximum. These observations support our previous hypothesis that changes in the relation of the depth of the variance maximum to the depth of the nutrocline may affect the rate of vertical nutrient input and, ultimately, primary and secondary production in the central North Pacific.

Variability of Water Column Transparency, Volume Flow and Suspended Solids Near San Diego Sewage Outfall (California): 15 Years of Data

Abstract The spatial and temporal variability of transparency measured for 15 years at 7 stations near the San Diego sewage outfall has been investigated and compared to the temporal variability of sewage suspended solid discharge and flow rate. the purpose of the time-series analyses was to distinguish natural from human (sewage discharge) causes of temporal changes in transparency. the results show that: 1) variations in transparency are highly correlated over the entire area, but there is a gradient in means and variability in the direction perpendicular to the coast; 2) there are no long term trends for increase or decrease in the water clarity at any of the stations; 3) most of the variance of transparency is contained in the seasonal frequency band; 4) over the same time period sewage discharge has significantly increased and suspended solids decreased; 5) most of the variance of these human-caused properties is in the interannual frequency band; (6) there is no correlation at any time-lag between w...