Some Observations on the Chlorophyll Maximum and Primary Production in the Eastern North Pacific

An oceanographic transect between Central California and the Hawaiian Islands delimited a continuous subsurface chlorophyll maximum associated with an oxygen maximum at 1% to 0.1% light penetration depths. The phytoplankton community consisted primarily of nannoplankton with a prominent diatom component, and was photosynthetically active as measured in „low light level”︁ incubators, contributing an estimated 20–30% to total water column production. Functioning as a nutrient trap blocking the vertical movement of nutrients into the upper euphotic zone, the chlorophyll maximum apparently required an overlying pycnocline for development.     

Primary production studies on a new reservior; Bighorn Lake-Yellowtail Dam,Montana, U.S.A. *

A definite algal succession pattern was established for all 3 years of study. Volume-based phytoplankton density and chlorophyll concentration decreased down-reservoir. However, the depth of the euphotic zone increased down-reservoir as silt settled out. Consequently, the euphotic zone standing crops were greatest mid-reservoir. Insufficient light penetration was established as the principal limiting factor to primary production in the upper end of the reservoir. Decreased primary production in the lower end of the reservoir did not appear to be due to nutrient limitation. Comparison of sampling periods common to all years of the study showed that estimated net primary production increased 84% in 1970 over 1968.     

An attempt to simulate the subsurface chlorophyll maximum using populations of unphased oscillating cells

A model is proposed to represent the vertical distribution ofchlorophyll in tropical waters. It assumes that the phytoplanktonpopulations are composed of cells which oscillate around a meanlevel at a density _t = B (close to the nutricline), with anamplitude A; A and B are normally distributed variables; theoscillations are unphased so that the number of cells in a waterlayer does not vary with the time. This model is tested on datafrom the eastern tropical Atlantic and from the southwesterntropical Pacific. The results agree with the observations whenthe surface mixed layer is nutrient-exhausted. However, themodel seems to account only for a deep population, and is thenconsistent with the hypothesis of a distinct mixed layer floralassociation. The agreement between the results and the observationsis difficult when the surface mixed layer is not nutrient-limited,in areas of upwelling or intense vertical mixing. The modelaccounts for a splitting of the subsurface chlorophyll maximumafter the passage of a front in the Cape Lopez area (GuineaGulf). It also accounts for the relations between the subsurfacechlorophyll maximum, the pycnocline, and the nutricline at 48stations from the cruise PREFIL 2 in the southwestern tropicalPacific. The permanent subsurface chlorophyll maximum of oligotrophicareas seems to be more satisfactorily explained by the unphasedoscillations invoked by the model than by the sinking of thecells. These unphased oscillations have not yet been observed;their possible consequences concerning the primary productionand grazing are postulated.     

Spatial Distribution, Structure, Biomass, and Physiology of Microbial Assemblages across the Southern Ocean Frontal Zones during the Late Austral Winter

We examined the spatial distributions of picoplankton, nanoplankton, and microplankton biomass and physiological state relative to the hydrography of the Southern Ocean along 90 degrees W longitude and across the Drake Passage in the late austral winter. The eastern South Pacific Ocean showed some large-scale biogeographical differences and size class variability. Microbial ATP biomass was greatest in euphotic surface waters. The horizontal distributions of microbial biomass and physiological state (adenylate energy charge ratio) coincided with internal currents (fronts) of the Antarctic Circumpolar Current. In the Drake Passage, the biological scales in the euphotic and aphotic zones were complex, and ATP, total adenylate, and adenylate energy charge ratio isopleths were compressed due to the extension of the sea ice from Antarctica and constriction of the Circumpolar Current through the narrow passage. The physiological state of microbial assemblages and biomass were much higher in the Drake Passage than in the eastern South Pacific Ocean. The temperature of Antarctic waters, not dissolved organic carbon, was the major variable controlling picoplankton growth. Estimates of picoplankton production based on ATP increments with time suggest that production under reduced predation pressure was 1 to 10 mug of carbon per liter per day. Our results demonstrate the influence of large-scale hydrographic processes on the distribution and structure of microplankton, nanoplankton, and picoplankton across the Southern Ocean.     

Marine primary production in the Canadian Arctic, 1956, 1961�C1963

Marine primary productivity studies pursued in the Canadian Arctic in 1961–1963, using standard techniques (oxygen and carbon-14) of the time, showed that maximum production developed rapidly in July at 5-m depth under very low light intensities and under intact but melting sea ice. The time of maximum production was correlated with depths of snow in spring. Low production levels were found in August in those years and estimated in 1956. Nitrate exhaustion occurred before maximum production and was followed by rapid sinking of chlorophyll and productivity and reduction of assimilation numbers. Gross production values greatly exceeded net production values in 1961–1963, and gross production in 2 years was about double that of a third year, which may be due to different initial conditions of nutrient concentrations in the euphotic zone. Interannual variability in primary production may be the result of changes in water masses in the surface layer, as driven by atmospheric pressure patterns.     

Mixotrophic nanoplankton in oligotrophic surface waters of the Sargasso Sea may employ phagotrophy to obtain major nutrients

The vertical distribution and abundance of mixotrophic nanoplanktonwas examined during two cruises to the Sargasso Sea south ofBermuda. Fluorescently labeled bacteria and cyanobacteria wereused as tracers of ingestion in experiments designed to determineabundances of mixotrophic nanoplankton. Phagotrophic nanoplanktonic(2–20 µm) algae ranged from undetectable to >100ml^–1, and were more abundant near the surface (up to 140ml^–1) than in the deeper euphotic zone. On two occasions,50% of the phototrophic nanoplankton in surface waters wereobserved with ingested fluorescent tracers. The contributionof mixotrophic algae to the total phototrophic nanoplanktonassemblage in the deep chlorophyll maximum, however, did notexceed 0.5%. It is possible that mixotrophic algae were moreabundant in the deep chlorophyll maximum, but were not phagotrophicallyactive. Two 4 day experimental incubations were subsequentlycarried out to examine the adaptive significance of phagotrophicbehavior for algae in surface waters of the Sargasso Sea. Greatermixotrophic nanoplankton abundances were observed in treatmentsthat received no nutrient inputs and were limited by the availabilityof inorganic nutrients during the experiments. A decrease inthe abundance of mixotrophic algae or a decrease in their phagotrophicactivity occurred with nutrient enrichment. Based on the experimentalresults, we suggest that phagotrophy was a mechanism by whichthese algae supplemented nutrient acquisition during periodsof low dissolved nutrient concentrations. Higher abundancesof mixotrophic nanoplankton observed in the upper 50 m of theSargasso Sea may have been due to the generally low nutrientconcentrations in these waters.     

The upper limit of photosynthetic productivity by phytoplankton: evidence from Ethiopian soda lakes

Photosynthetic production by phytoplankton was studied in two Ethiopian soda lakes, alkalinity 51-67 m-equiv./l, with abundant blue-green algae. The deeper lake, L. Aranguadi, contained extremely dense crops composed almost entirely of SpiruUna (Oscillatoria, Arthrospira) platensis. Measurements of the spectral attenuation of light showed that the most penetrating component was displaced to the red spectral region, and estimates of the chlorophyll a content in umt area of the euphotic zone were often similar to the highest values (～200-300 mg/m²) expected on theoretical grounds. The vertical distribution of photosynthetic activity per unit water volume was of a typical pattern for phytoplankton, with light-inhibition often present, although the euphotic zones did not exceed 0.6 m in depth. From these profiles, computed rates of gross photosynthesis per unit area of lake surface reached a magnitude of 1.4–2.4 g O₂/m².h in both lakes. They were obviously severely limited by self-shading behaviour i n the algal populations. An appreciable inverse relationship between photosynthetic capacity and population density was only found in the densest populations of L. Aranguadi, with chlorophyll a content > 2000 mg/m³, where depressed rates might result from experimental artefacts in closed bottles. In this lake, two independent estimates of gross production, based on the analysis of diurnal changes in the open water, were as high as 43 and 57 g O₂/m². day. Diurnal changes of stratification in the two lakes are described and related to the controlling temperature (density) stratification. They include occasionally complete nocturnal deoxygenation in the deeper lake, evidence of heavy respiratory uptake. The high photosynthetic productivity is interpreted as dependent upon the coupling of high algal contents in the euphotic zone (^S) with high values of photosynthetic capacity (^max)- It is probably favoured by the tropical situation affecting temperature and illumination, by a surplus of dissolved inorganic phosphate, and especially by the considerable reserves of carbon dioxide in these soda lakes.     

Global distribution patterns of distinct clades of the photosynthetic picoeukaryote Ostreococcus

Ostreococcus is a marine picophytoeukaryote for which culture studies indicate there are ‘high-light'' and ‘low-light'' adapted ecotypes. Representatives of these ecotypes fall within two to three 18S ribosomal DNA (rDNA) clades for the former and one for the latter. However, clade distributions and relationships to this form of niche partitioning are unknown in nature. We developed two quantitative PCR primer-probe sets and enumerated the proposed ecotypes in the Pacific Ocean as well as the subtropical and tropical North Atlantic. Statistical differences in factors such as salinity, temperature and NO₃ indicated the ecophysiological parameters behind clade distributions are more complex than irradiance alone. Clade OII, containing the putatively low-light adapted strains, was detected at warm oligotrophic sites. In contrast, Clade OI, containing high-light adapted strains, was present in cooler mesotrophic and coastal waters. Maximal OI abundance (19 555±37 18S rDNA copies per ml) was detected in mesotrophic waters at 40 m depth, approaching the nutricline. OII was often more abundant at the deep chlorophyll maximum, when nutrient concentrations were significantly higher than at the surface (stratified euphotic zone waters). However, in mixed euphotic-zone water columns, relatively high numbers (for example, 891±107 18S rDNA copies per ml, Sargasso Sea, springtime) were detected at the surface. Both Clades OI and OII were found at multiple euphotic zone depths, but co-occurrence at the same geographical location appeared rare and was detected only in continental slope waters. In situ growth rate estimates using these primer-probes and better comprehension of physiology will enhance ecological understanding of Ostreococcus Clades OII and OI which appear to be oceanic and coastal clades, respectively.     

Movement patterns for a critically endangered species, the leatherback turtle (Dermochelys coriacea), linked to foraging success and population status

Foraging success for pelagic vertebrates may be revealed by horizontal and vertical movement patterns. We show markedly different patterns for leatherback turtles in the North Atlantic versus Eastern Pacific, which feed on gelatinous zooplankton that are only occasionally found in high densities. In the Atlantic, travel speed was characterized by two modes, indicative of high foraging success at low speeds (<15 km d(-1)) and transit at high speeds (20-45 km d(-1)). Only a single mode was evident in the Pacific, which occurred at speeds of 21 km d(-1) indicative of transit. The mean dive depth was more variable in relation to latitude but closer to the mean annual depth of the thermocline and nutricline for North Atlantic than Eastern Pacific turtles. The most parsimonious explanation for these findings is that Eastern Pacific turtles rarely achieve high foraging success. This is the first support for foraging behaviour differences between populations of this critically endangered species and suggests that longer periods searching for prey may be hindering population recovery in the Pacific while aiding population maintenance in the Atlantic.     

Summer nutrient dynamics of the Middle Atlantic Bight: nitrogen uptake and regeneration

Nitrate and ammonium uptake and ammonium regeneration rates(by zooplankton, microplankton and benthos) were measured onthe Atlantic continental shelf (Middle Atlantic Bight) duringsummer, 1980. Euphotic zone profiles of NO₃^– and NH₄⁺uptake rates were similar in magnitude and vertical structureover a large geographical area. Microplankton NH₄⁺ regenerationrates, although measured less frequently, also showed a relativelyconsistent vertical structure; rates were positively correlatedwith uptake rates. Nitrate assimilation (‘new’ production)was used to estimate vertical eddy diffusivity and paniculatesinking rates. Eddy diffusion estimates ranged from <0.1to >2.0 cm² s^–1 and were positively related to arealprimary production. Estimated particulate sinking rates averaged5 mg at Nm^–2d^–1 and compared favorably with sedimentationrates measured from free-floating and moored sediment traps.Benthic nitrogen regeneration rates represented <10% of thispaniculate nitrogen flux. Within the mixed layer, NH₄⁺ assimilation(‘regenerated’ production) represented 50–80%of the total (NO₃^– + NH₄⁺ ) nitrogen productivity and33% for the euphotic zone. Of this, 30% was attributed to zooplankton,63% to microplankton (<100 µm) and 7% to benthos. Onthe average, 74% of the microplankton NH₄⁺ regeneration wasassociated with organisms passing 1 µm filters.     

Summer depth selection in crustacean zooplankton in nutrient‐poor boreal lakes is affected by recent residential development

1. Strong vertical gradients in light, water temperature, oxygen, algal concentration and predator encounters during summer in stratified lakes may influence patterns of depth selection in crustacean zooplankton, especially Daphnia species. 2. To test how crustacean depth selection varies among lakes along a gradient of catchment disturbance by recent residential development and land use change, we calculated the weighted mean depth distribution of the biomass of crustaceans by day and night in eight nutrient‐poor boreal lakes. 3. Generally, the greatest biomass of crustaceans was located at the metalimnion or at the lower boundary of the euphotic zone during thermal stratification in July. The crustacean zooplankton avoided warm surface layers and tended to stay in colder deep waters by both day and night. They also remained at greater depths in lakes with a more extensive euphotic zone. 4. There was some evidence of upward nocturnal migrations of large Daphnia and copepods in some lakes, and one case of downward migration in a lake inhabited by chaoborid larvae. 5. Multivariate regression trees (MRT) were used to cluster crustaceans and Daphnia species in homogeneous groups based on lake natural and disturbance factors. For crustaceans, the depth of the euphotic zone, the sampling depth (epilimnion, metalimnion and hypolimnion), time (day or night) of sampling and the biomass of chlorophyll a were the main driving factors. For Daphnia species, the drainage area, the sampling depth, the cleared land surface area within the catchment and the concentration of total dissolved phosphorus were the main factors.     

Clouds, deep chlorophyll maxima and the nutrient supply to the mixed layer of stratified water bodies

A deep chlorophyll maximum (DCM) is common in the pycnoclineof the offshore tropical and subtropical oceans outside of divergencesas well as in temperate seas and lakes during summer. It trapsthe diffusive flux of nutrients into the depleted upper layers.When the DCM and the associated nutricline are situated in thetop of the pycnocline (e.g. as during early summer in temperateseas), cloudiness materially reducing photosynthesis in theDCM may lead to a nutrient flux into the mixed layer. For thewarm seas, with the DCM and the nutricline being usually deepin the stratified domain, it is shown here that even severecloud cover can only be expected to lead to enhanced nutrientflux into the upper parts of the pycnocline but not the mixedlayer. The same is likely for temperate seas and deep lakesduring the height of the summer. The implication of irregular,enhanced upward flux of nitrate for the ratio of new/regeneratedprimary production is noted.     

Modelling the vertical distribution of Prochlorococcus and Synechococcus in the North Pacific Subtropical Ocean

A simple model was developed to examine the vertical distribution of Prochlorococcus and Synechococcus ecotypes in the water column, based on their adaptation to light intensity. Model simulations were compared with a 14-year time series of Prochlorococcus and Synechococcus cell abundances at Station ALOHA in the North Pacific Subtropical Gyre. Data were analysed to examine spatial and temporal patterns in abundances and their ranges of variability in the euphotic zone, the surface mixed layer and the layer in the euphotic zone but below the base of the mixed layer. Model simulations show that the apparent occupation of the whole euphotic zone by a genus can be the result of a co-occurrence of different ecotypes that segregate vertically. The segregation of ecotypes can result simply from differences in light response. A sensitivity analysis of the model, performed on the parameter alpha (initial slope of the light-response curve) and the DIN concentration in the upper water column, demonstrates that the model successfully reproduces the observed range of vertical distributions. Results support the idea that intermittent mixing events may have important ecological and geochemical impacts on the phytoplankton community at Station ALOHA.     

Counterintuitive effects of global warming‐induced wind patterns on primary production in the Northern Humboldt Current System

It has been hypothesized that global warming will strengthen upwelling‐favorable winds in the Northern Humboldt Current System (NHCS) as a consequence of the increase of the land–sea thermal gradient along the Peruvian coast. The effect of strengthened winds in this region is assessed with the use of a coupled physical–biogeochemical model forced with projected and climatological winds. Strengthened winds induce an increase in primary production of 2% per latitudinal degree from 9.5°S to 5°S. In some important coastal upwelling sites primary production is reduced. This is due to a complex balance between nutrient availability, nutrient use efficiency, as well as eddy‐ and wind‐driven factors. Mesoscale activity induces a net offshore transport of inorganic nutrients, thus reducing primary production in the coastal upwelling region. Wind mixing, in general disadvantageous for primary producers, leads to shorter residence times in the southern and central coastal zones. Overall, instead of a proportional enhancement in primary production due to increased winds, the NHCS becomes only 5% more productive (+5 mol C m^?2 year^?1), 10% less limited by nutrients and 15% less efficient due to eddy‐driven effects. It is found that regions with a initial strong nutrient limitation are more efficient in terms of nutrient assimilation which makes them more resilient in face of the acceleration of the upwelling circulation.     

Eddy diffusion as a possible determinant of vertical nitrate and chlorophyll distributions in the Mediterranean Sea

Using coupled biological and physical submodels, simulated distributionsof nitrate and chlorophyll agree well with summer observations,but are very sensitive to vertical velocity profiles and calculatedrates of eddy diffusion.     

Fine-scale spatial patterns in the coastal epiplankton off southern California

Fine-scale vertical (5 – 40 m) and horizontal (50 –500 m) patterns of temperature, chlorophyll and abundance ofzooplankton species were sampled with a pump filtration systemin the surface waters offshore of San Diego in May and October,1978. Intense and consistent patterns were most apparent invertical profiles. Herbivorous zooplankton were more consistentlyassociated with the estimated primary productivity maximum thanwith the deeper chlorophyll maximum layer, which representeda phytoplankton biomass maximum. Predators were positively correlatedwith abundant potential prey species. Variations in body lengthwith depth suggest that these fine-scale patterns were sufficientlystable to influence zooplankton growth. Consequences for grazingand predator – prey interactions in pelagic ecosystemsare discussed. ¹Present address: NOAA/NMFS Southwest Fisheries Center, PO Box271, La Jolla, CA 92038, USA     

Implications of seasonal mixing for phytoplankton production and bloom development

Based on a 1D model considering phytoplankton and nutrients in a vertical water column, we investigate the consequences of temporal and spatial variations in turbulent mixing for phytoplankton production and biomass. We show that in seasonally mixed systems, the processes controlling phytoplankton production and the sensitivity of phytoplankton abundance to ambient light, trophic state and mixed-layer depth differ substantially from those at steady state in systems with time-constant diffusivities. In seasonally mixed systems, the annually replenished nutrient pool in the euphotic zone is an important factor for phytoplankton production supporting bloom development, whereas without winter mixing, production mainly depends on the diffusive nutrient flux during stratified conditions. Seasonal changes in water column production are predominantly determined by seasonal changes in phytoplankton abundance, but also by seasonal changes in specific production resulting from the transport of nutrients, the exploitation of the nutrient pool and the increase in light shading associated with phytoplankton growth. The interplay between seasonal mixing and the vertical distribution of mixing intensities is a key factor determining the relative importance of the processes controlling phytoplankton production and the sensitivity of the size and timing of the annual maximum phytoplankton abundance to the abiotic conditions.