Thomas L. Hayward

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.

Oxygen supersaturation in the ocean: biological versus physical contributions.

A method based on measurements of dissolved molecular nitrogen, molecular oxygen, and argon can distingish biological from physical contributions to oxygen supersaturation in the ocean. The derived values of biological O2 production can be used as a check on estimates of total organic productivity measured by instantaneous rates of carbon-14 assimilation. Application to the shallow summer O2 maxima in the North Pacific gyres shows that about 72% of the O2 supersaturation maximum at 28�N and about 86% of the maximum at 40�N are due to net photosynthetic production.

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.

The nutrient distribution and primary production in the central North Pacific

Abstract The relation between horizontal and vertical nutrient distributions and biological pattern in the North Pacific central gyre suggests that new primary is strongly nutrient limited and that the nutrient supply rate is a major determinant of biological pattern. However, no consistent relation between the nutrient distribution and biological pattern is evident on any spatial or temporal scale. Along- and cross-isopycnal nutrient fluxes to the euphotic zone, estimated from nutrient gradients and eddy diffusivities predicted from microstructure and tracer distributions, are roughly equivalent, but both are a small fraction of the nutrient requirement. This suggests that mixing does not follow Fickian diffusion and that variation in the nature of mixing, rather thann in the nutrient distribution itself, regulate the nutrient input to the euphotic zone and biological pattern. It is difficult to account for the maintenanceof the observed staady-state vertical structure in nutrients and biological properties, and, at the same time, to reconcile the great depth of the nutricline with evidence of considerable new production at much shallower depths. The latter appears to require a significant nutrient flux across a 30–50 m depth zone with a near-zero concentration gradient. An understanding of the processes regulating primary production in the open ocean will await a better understanding of mixing, and a complete model must be able to account for the maintenance of steady-state vertical structure and reconcile the discrepancy between estimates of the nutrient supply and uptake rates.

Pacific Ocean climate change: atmospheric forcing, ocean circulation and ecosystem response

A major climate change event that affected atmospheric forcing, ocean circulation and ecosystem structure of the Pacific Ocean began in the mid-1970s. Changes in biomass, and presumably productivity, of the lower trophic levels (phytoplankton and Zooplankton) were directly attributed to this event. It also appears that some individual species at higher trophic levels were influenced, but cause-and-effect relationships are more difficult to document at the species level. Recent work shows that at least five major pelagic ecosystems responded to this event, but in different ways, and both increases and decreases in biomass were seen. Changes of this magnitude are well documented in the paleo-oceanographic record. However, it remains to be determined to what extent the changes were caused by natural cycles versus anthropogenic change (global warming).

Nearsurface pattern in the California Current: coupling between physical and biological structure

Abstract The results of a four-year program of continuous nearsurface mapping and a 12-year time series of nearsurface chlorophyll measurements, both made by the CalCOFI program, are interpreted in the context of the CalCOFI hydrographic time series. There are consistent regional differences in the mechanisms affecting nutrient input to the upper layers, and these result in different spatial and temporal patterns of chlorophyll concentration. We define three regimes in which nearsurface ecosystem structure appears to be regulated differently. A sharp physical and biological front at the inner edge of the low-salinity California Current jet separates offshore and inshore regions. The inshore area is further divided into northern and southern regions based upon the intensity of isopycnal shoaling, which alters the relationship between physical structure and chlorophyll concentration. These observations indicate that even on the scale of the CalCOFI area a regional approach to developing models and interpreting mesoscale structure will improve our understanding of important physical–biological interactions.

Primary production in the North Pacific Central Gyre: A controversy with important implications

Photosynthesis in the ocean is measured by (14)C-CO(2) uptake in bottle incubations. It can also be estimated from the distribution of biogeochemical properties. In the subtropical gyres, the biogeochemical method yields a rate that is apparently greater than that measured by the (14)C technique. This led to the controversial suggestion that oceanic primary production has been substantially underestimated. If correct, this suggestion also requires that there be a larger input of nitrogen and phosphorus, and it would make it difficult to explain the observed steady-state vertical distribution of properties. Resolution of the controversy is likely to be of broad interest to oceanographers because it may also lead to new insights into oceanic mixing and the cycling of carbon and nitrogen.

The shallow oxygen maximum layer and primary production

Abstract The shallow oxygen maximum (SOM) is a vertical maximum in the dissolved oxygen concentration of the upper ocean. This feature develops largely from photosynthetic oxygen production, and measurement of the rate of oxygen accumulation provides the basis to make an independent estimate of primary production. Net accumulation of dissolved oxygen represents new production, which is generally presumed to require a new nutrient input consisting of some form of preformed nutrients. Here the time scales, vertical distribution or properties and new nutrient sources associated with formation of the SOM in the North Pacific ocean area considered. The SOM forms on two distinct time scales, episodic and gradual, and apparently through different processes on each scale. The episodic SOM develops from physical events in which nutricline water is transported into the mixed layer and then ventilated to the atmosphere. This process results in the local creation of preformed nutrients that stimulate a phytoplankton bloom and a vertical maximum of chlorophyll and dissolved oxygen. However, the time scale of formation of the gradual SOM is inconsistent with this mechanism because repeated mixing and ventilation events cannot result in the gradual accumulation of oxygen. An alternative new nutrient source is required. It is hypothesized that the gradual SOM develops from input of the “new” nutrients contained in organic matter transported from other depths by migrating animals and/or sinking particles. This hypothesis implies that the concept of new production must be carefully defined in context of the vertical structure of the euphotic zone. Whether or not my hypothesis is correct, these observations show that estimating primary production by measuring the increase in dissolved oxygen is very difficult. For both the episodic and gradual SOM, estimating production requires that the gas fluxes due to air-sea exchange and vertical and horizontal mixing be well known, in addition to direct measurements of the temporal change in dissolved oxygen within a water parcel.

Chemical tracers of biological processes in shallow waters of North Pacific: Preformed nitrate distributions

Distributions of nitrate and Apparent Oxygen Utilization in the upper subtropical North Pacific Ocean reveal a layer with negative values of preformed nitrate. This layer occurs at depths just below the 1% light level and above the density of sigma theta 25.6. We show that large-scale spatial patterns in the distribution of this feature are determined by an interaction between light penetration and the depth of isopycnal surfaces which are ventilated in nutrient rich surface waters. Although the data alone are insufficient to distinguish between several possible causes, we believe the geographic and depth distributions of the negative preformed nitrate feature are most readily explained by respiration of nitrogen-poor dissolved organic matter (DOM) from the surface ocean with the possible accompaniment of nitrate uptake. Dissolved organic carbon gradients and transport calculations suggest that a significant fraction of the carbon flux out of the euphotic zone may be via DOM, indicating that the processes responsible for creating the negative preformed nitrate feature could alter the metabolite stoichiometry in upper subtropical Pacific Ocean.

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.