Implications For Primary Productivity Research Articles

Distinct drivers of two size fractions of operationally dissolved iron in a temperate river

AbstractRiverine dissolved iron (Fe) affects water color, nutrients, and marine carbon cycling. Fe size and coupling with dissolved organic matter (DOM), in part, modulates the biogeochemical roles of riverine Fe. We used size fractionation to operationally define dissolved Fe (< 0.22 μm) into soluble (< 0.02 μm) and colloidal (0.02–0.22 μm) fractions in order to characterize the downstream drivers, concentrations, and fluxes of Fe across season and hydrologic regime at the freshwater Connecticut River mainstem, which we sampled bi‐weekly for 2 yrs. Drivers of colloidal and soluble Fe concentrations were markedly different. The response of colloidal Fe concentration to changes in discharge was modulated by water temperature; colloidal Fe decreased with increasing discharge at temperatures < 10.5°C, but increased with increasing discharge at temperatures > 10.5°C. Conversely, soluble Fe concentrations were only positively correlated to discharge at high temperatures (> 20°C). Soluble Fe was strongly positively correlated to a humic‐like DOM fluorescence component, suggesting coupling with DOM subsets, potentially through complexation. While average colloidal Fe fluxes varied twofold seasonally, soluble Fe fluxes varied ninefold; therefore, soluble Fe variability was more important to the overall dissolved Fe variability than colloidal Fe, despite lower concentrations. Seasonal Fe fluxes were decoupled from discharge: dissolved and soluble Fe fluxes were greatest in the fall, whereas discharge was greatest in the spring. Fluxes of soluble Fe, which may be more bioavailable and more likely to be exported to the ocean, were lowest in the summer when downstream biological demand is high, having implications for primary productivity and iron uptake.

Projected climate variability of internal waves in the Andaman Sea

The Andaman Sea, in the northeast Indian Ocean, is renowned for large-amplitude internal waves. Here, we use a global climate model (CanESM5) to investigate the long-term variability of internal waves in the Andaman Sea under a range of shared socioeconomic pathway (SSP) scenarios. SSPs are future societal development pathways related to emissions and land use scenarios. We project that mean values of depth-averaged stratification will increase by approximately 6% (SSP1-2.6), 7% (SSP2-4.5), and 12% (SSP5-8.5) between 1871-1900 and 2081-2100. Simulating changes in internal tides between the present (2015-2024) and the end-century (2091-2100), we find that the increase in stratification will enhance internal tide generation by approximately 4 to 8%. We project that the propagation of internal tides into the Andaman Sea and the Bay of Bengal will increase by 8 to 18% and 4 to 19%, respectively, under different SSP scenarios. Such changes in internal tides under global warming will have implications for primary production and ecosystem health not only in the Andaman Sea but also in the Bay of Bengal.

Molecular mechanisms underlying iron and phosphorus co-limitation responses in the nitrogen-fixing cyanobacterium Crocosphaera.

In the nitrogen-limited subtropical gyres, diazotrophic cyanobacteria, including Crocosphaera, provide an essential ecosystem service by converting dinitrogen (N2) gas into ammonia to support primary production in these oligotrophic regimes. Natural gradients of phosphorus (P) and iron (Fe) availability in the low-latitude oceans constrain the biogeography and activity of diazotrophs with important implications for marine biogeochemical cycling. Much remains unknown regarding Crocosphaera's physiological and molecular responses to multiple nutrient limitations. We cultured C. watsonii under Fe, P, and Fe/P (co)-limiting scenarios to link cellular physiology with diel gene expression and observed unique physiological and transcriptional profiles for each treatment. Counterintuitively, reduced growth and N2 fixation resource use efficiencies (RUEs) for Fe or P under P limitation were alleviated under Fe/P co-limitation. Differential gene expression analyses show that Fe/P co-limited cells employ the same responses as single-nutrient limited cells that reduce cellular nutrient requirements and increase responsiveness to environmental change including smaller cell size, protein turnover (Fe-limited), and upregulation of environmental sense-and-respond systems (P-limited). Combined, these mechanisms enhance growth and RUEs in Fe/P co-limited cells. These findings are important to our understanding of nutrient controls on N2 fixation and the implications for primary productivity and microbial dynamics in a changing ocean.

Prokaryotic responses to a warm temperature anomaly in northeast subarctic Pacific waters

Recent studies on marine heat waves describe water temperature anomalies causing changes in food web structure, bloom dynamics, biodiversity loss, and increased plant and animal mortality. However, little information is available on how water temperature anomalies impact prokaryotes (bacteria and archaea) inhabiting ocean waters. This is a nontrivial omission given their integral roles in driving major biogeochemical fluxes that influence ocean productivity and the climate system. Here we present a time-resolved study on the impact of a large-scale warm water surface anomaly in the northeast subarctic Pacific Ocean, colloquially known as the Blob, on prokaryotic community compositions. Multivariate statistical analyses identified significant depth- and season-dependent trends that were accentuated during the Blob. Moreover, network and indicator analyses identified shifts in specific prokaryotic assemblages from typically particle-associated before the Blob to taxa considered free-living and chemoautotrophic during the Blob, with potential implications for primary production and organic carbon conversion and export.

Average cosine coefficient and spectral distribution of the light field under sea ice: Implications for primary production

The Arctic spring phytoplankton bloom has been reported to commence under a melting sea ice cover as transmission of photosynthetically active radiation (PAR; 400–700 nm) suddenly increases with the formation of surface melt ponds. Spatial variability in ice surface characteristics, i.e., snow thickness or melt pond distributions, and subsequent impact on transmitted PAR makes estimating light-limited primary production difficult during this time of year. Added to this difficulty is the interpretation of data from various sensor types, including hyperspectral, multispectral, and PAR-band irradiance sensors, with either cosine-corrected (planar) or spherical (scalar) sensor heads. To quantify the impact of the heterogeneous radiation field under sea ice, spectral irradiance profiles were collected beneath landfast sea ice during the Green Edge ice-camp campaigns in May–June 2015 and June–July 2016. Differences between PAR measurements are described using the downwelling average cosine, μd, a measure of the degree of anisotropy of the downwelling underwater radiation field which, in practice, can be used to convert between downwelling scalar, E0d, and planar, Ed, irradiance. A significantly smallerμd(PAR) was measured prior to snow melt compared to after (0.6 vs. 0.7) when melt ponds covered the ice surface. The impact of the average cosine on primary production estimates, shown in the calculation of depth-integrated daily production, was 16% larger under light-limiting conditions when E0d was used instead of Ed. Under light-saturating conditions, daily production was only 3% larger. Conversion of underwater irradiance data also plays a role in the ratio of total quanta to total energy (EQ/EW, found to be 4.25), which reflects the spectral shape of the under-ice light field. We use these observations to provide factors for converting irradiance measurements between irradiance detector types and units as a function of surface type and depth under sea ice, towards improving primary production estimates.

Fe and Nutrients in Coastal Antarctic Streams: Implications for Primary Production in the Ross Sea

AbstractThe Southern Ocean (SO) has been an area of biogeochemical interest due to the presence of macronutrients (N, P, and Si) but lack of the expected primary production response, which is thought to be primarily due to Fe limitation. Because primary production is associated with increased drawdown of atmospheric CO2, it is important to quantify the fluxes of Fe and other nutrients into the SO. Here we present data from subaerial streams that flow into the Ross Sea, a sector of the coastal SO. Water samples were collected in the McMurdo Dry Valleys, Antarctica, and analyzed for macronutrients and Fe to determine the potential impact of terrestrial water input on the biogeochemistry of coastal oceanic waters. The physiochemical forms of Fe were investigated through analysis of three operationally defined forms: acid‐dissolvable Fe (no filtration), filterable Fe (<0.4 μm), and dissolved Fe (<0.2 μm). The combined average flux from two McMurdo Dry Valley streams was approximately 240 moles of filterable Fe per year. The dissolved fraction of Fe made up 18%–27% of the filterable Fe. The stream data yield an average filterable stoichiometry of N3P1Si100Fe0.8, which is substantially different from the planktonic composition and suggests that these streams are a potential source of Fe and P, relative to N and Si, to coastal phytoplankton communities. While the Fe flux from these streams is orders of magnitude less than estimated eolian and iceberg sources, terrestrial streams are expected to become a more significant source of Fe to the Ross Sea in the future.

Factors affecting trophic compositions of offshore benthic invertebrates at a sub‐Antarctic archipelago

AbstractFood availability is a key regulator of the distribution, metabolism, and success of benthic populations. In deep sea ecosystems, hydrodynamics and depth play fundamental roles in determining benthic food resources. Recent studies suggest that the Southern Ocean sub‐Antarctic front is shifting southward, with implications for primary production and food availability around the sub‐Antarctic Islands embedded in the Antarctic Circumpolar Current. We used fatty acid (FA) and stable isotope (SI) analyses to investigate the trophic pattern of benthic invertebrates and suspended particulate matter (SPM) at three Depths (shallow: 100 m, middle: 300 m, and deep: 600 m) in three hydrographic regions with different flow and productivity regimes around the Prince Edward Islands. Both region and depth affected the SI values of SPM, while feeding guild was the key factor influencing consumer trophic values. Depth affected the δ15N of all trophic groups and the FA compositions of suspension feeders. Deeper samples had a higher δ15N and showed a greater proportion of mono‐ and saturated fatty acids, reflecting greater remineralization of SPM and of food reaching the seafloor. Region affected the δ13C and FA values of SPM, suspension feeders and deposit feeder/scavengers, with differences between the interisland and open ocean regions. This was probably linked to the retention of nutrients and phytoplankton between the islands. Critically, the effects of depth and hydrographic region were taxon specific, indicating that long‐term responses to environmental change may have complex consequences for the feeding ecology and viability of benthic populations, with implications for the higher trophic levels that these populations support.

Atmospheric iron fluxes in the northern region of the Gulf of California: Implications for primary production and potential Fe limitation

To study the temporal variability of atmospheric mineral dust and Fe fluxes to the northern region of the Gulf of California, dust samples were collected at San Felipe, Baja California (Site 1) and Puerto Peñasco, Sonora (Site 2), from May 2010 to December 2011. Dust fluxes were partially associated with monsoon circulation, with highest (38 ± 20mgm−2 d−1) and lowest (8.8 ± 4.9mgm−2 d−1) fluxes linked to southwesterly winds (monsoon season) and north-northwesterly winds (non-monsoon season), respectively. Our analysis suggests that the surrounding deserts are the most probable source of dust arriving into the Gulf of California. However, analyses of Al and Fe concentrations in dusts showed no trends that could identify specific particulate Fe provenance. Average particulate Fe atmospheric fluxes (FeAtm) showed no clear temporal trends and their magnitudes (3.2 ± 3.4 and 6 ± 10µmolm−2 d−1 for sites 1 and 2, respectively) could be considered medium-to-low in magnitude within a global context. An Fe limitation index (FeLI), calculated as the ratio of phytoplankton Fe requirements to atmospheric and upwelling Fe fluxes, is proposed to estimate the impact of atmospheric mineral dust on phytoplankton primary production in surface waters of the Gulf of California. On a seasonal time scale, FeLI results suggest that under winter conditions, there is no evidence of Fe limitation because upwelling Fe contribution (FeUp) is high enough to support primary production. In contrast, during summer conditions, when FeUp is very low or negligible, high FeAtm combined with high particulate Fe dissolution factors could prevent the northern Gulf of California from becoming Fe-limited. Finally, we postulate that at an interannual scale, conditions prevailing during ENSO events could increase atmospheric Fe fluxes to the Gulf of California, further contributing to prevent Fe limitation in this marginal sea.

Increased Precipitation and Nitrogen Alter Shrub Architecture in a Desert Shrubland: Implications for Primary Production.

Shrublands are one of the major types of ecosystems in the desert regions of northern China, which is expected to be substantially more sensitive to global environmental changes, such as widespread nitrogen enrichment and precipitation changes, than other ecosystem types. However, the interactive effects of nitrogen and precipitation on them remain poorly understood. We conducted a fully factorial field experiment simulating three levels of precipitation (ambient, +20%, +40%) and with two levels of nitrogen deposition (ambient, 60 kg N ha-1 yr-1) in a desert shrubland in the Mu Us Desert of northern China. We used plant architectural traits (plant cover, volume, twig size and number) as proxies to predict aboveground net primary productivity (ANPP) of the dominant shrub (Artemisia ordosica Krasch), and assessed the responses of plant productivity and architectural traits to water and nitrogen addition. We found significant differences in twig size and number of A. ordosica under water and nitrogen treatments but not in shrub cover/volume, which suggest that twig size and number of the shrub species were more sensitive to environmental changes. The productivity of the overall community was sensitive to increased precipitation and nitrogen, and shrubs played a more important role than herbaceous plants in driving productivity in this ecosystem. Precipitation- and nitrogen-induced increases in vegetation production were positively associated with increases in twig size and number of the dominant shrub. Water addition enhanced the twig length of A. ordosica, while nitrogen addition resulted in increased twig density (the number of twigs per square meter). Water and nitrogen interacted to affect twig length, but not twig number and shrub ANPP. The trade-off, defined as negative covariance between twig size and number, was likely the mechanism underlying the responses of twig length and shrub ANPP to water and nitrogen interactions. Our results highlight the sensitivity of twig size and number as indicators to estimate shrub production and the mechanism underpinning desert shrub ANPP response to global environmental changes.

Response of Soil Properties and Microbial Communities to Agriculture: Implications for Primary Productivity and Soil Health Indicators.

Agricultural intensification is placing tremendous pressure on the soil’s capacity to maintain its functions leading to large-scale ecosystem degradation and loss of productivity in the long term. Therefore, there is an urgent need to find early indicators of soil health degradation in response to agricultural management. In recent years, major advances in soil meta-genomic and spatial studies on microbial communities and community-level molecular characteristics can now be exploited as ‘biomarker’ indicators of ecosystem processes for monitoring and managing sustainable soil health under global change. However, a continental scale, cross biome approach assessing soil microbial communities and their functional potential to identify the unifying principles governing the susceptibility of soil biodiversity to land conversion is lacking. We conducted a meta-analysis from a dataset generated from 102 peer-reviewed publications as well as unpublished data to explore how properties directly linked to soil nutritional health (total C and N; C:N ratio), primary productivity (NPP) and microbial diversity and composition (relative abundance of major bacterial phyla determined by next generation sequencing techniques) are affected in response to agricultural management across the main biomes of Earth (arid, continental, temperate and tropical). In our analysis, we found strong statistical trends in the relative abundance of several bacterial phyla in agricultural (e.g., Actinobacteria and Chloroflexi) and natural (Acidobacteria, Proteobacteria, and Cyanobacteria) systems across all regions and these trends correlated well with many soil properties. However, main effects of agriculture on soil properties and productivity were biome-dependent. Our meta-analysis provides evidence on the predictable nature of the microbial community responses to vegetation type. This knowledge can be exploited in future for developing a new set of indicators for primary productivity and soil health.

Spatio-temporal variation in density of microphytoplankton genera in two tropical coral reefs of Mauritius

Spatial and temporal (seasonal and interannual) variation in microphytoplankton was investigated at two tropical coral reef sites off Mauritius (Belle Mare [BM] and Flic-en-Flac [FEF]), from 2010 to 2012. Each site was divided into three zones: coast, lagoon and reef. Microphytoplankton, nutrient and chlorophyll a samples were collected, and physico-chemical parameters were measured in situ. A total of 56 microphytoplankton genera were identified (43 diatoms, 8 dinoflagellates and 5 cyanobacteria). Total microphytoplankton density exhibited spatial and temporal variation, being higher at BM and near the coast, and mostly high in summer. Nutrient concentrations were highest near the coast as a result of run-off events and anthropogenic inputs, especially during heavy rainfall, and this resulted in elevated total microphytoplankton density. There were no clear diversity patterns at the two sites or within the different zones. At both sites, Navicula, Cocconeis and Fragilaria were the dominant diatoms, and Gymnodinium, Alexandrium and Protoperidinium were the dominant dinoflagellates. There was a shift in dominance from Navicula in summer to Fragilaria in winter. This seasonal shift may reflect how these species respond differently to environmental change, which could have implications for primary production at spatial and temporal scales.

Metaproteomics reveals differential modes of metabolic coupling among ubiquitous oxygen minimum zone microbes.

Marine oxygen minimum zones (OMZs) are intrinsic water column features arising from respiratory oxygen demand during organic matter degradation in stratified waters. Currently OMZs are expanding due to global climate change with resulting feedback on marine ecosystem function. Here we use metaproteomics to chart spatial and temporal patterns of gene expression along defined redox gradients in a seasonally stratified fjord to better understand microbial community responses to OMZ expansion. The expression of metabolic pathway components for nitrification, anaerobic ammonium oxidation (anammox), denitrification, and inorganic carbon fixation were differentially expressed across the redoxcline and covaried with distribution patterns of ubiquitous OMZ microbes including Thaumarchaeota, Nitrospina, Nitrospira, Planctomycetes, and SUP05/ARCTIC96BD-19 Gammaproteobacteria. Nitrification and inorganic carbon fixation pathways affiliated with Thaumarchaeota dominated dysoxic waters, and denitrification, sulfur oxidation, and inorganic carbon fixation pathways affiliated with the SUP05 group of nitrate-reducing sulfur oxidizers dominated suboxic and anoxic waters. Nitrifier nitrite oxidation and anammox pathways affiliated with Nirospina, Nitrospira, and Planctomycetes, respectively, also exhibited redox partitioning between dysoxic and suboxic waters. The numerical abundance of SUP05 proteins mediating inorganic carbon fixation under anoxic conditions suggests that SUP05 will become increasingly important in global ocean carbon and nutrient cycling as OMZs expand.

Variability in the sub-surface light climate at ecohydrodynamically distinct sites in the North Sea

This paper shows that the sub-surface light regime in the offshore North Sea varies spatially and seasonally between different ecohydrodynamic regions, which is likely to have important implications for primary production and carbon and nutrient fluxes in different areas of the North Sea. Measurements of downward irradiance were collected using different instruments (i.e. water column-profiling instruments, semi-autonomous moorings, and remote sensing) at three ecohydrodynamically distinct sites in the North Sea: in the southern Bight (SB), at the Oyster Grounds (OG) and north of the Dogger Bank (ND). The ND site was the deepest, and had the lowest and least variable light attenuation coefficients (mean Kd(PAR) = 0.11 m−1). The onset of the phytoplankton spring bloom was earlier than at the other two sites. In summer, ND had low Kd(PAR) ~ 0.07 m−1 and light penetration was shifted towards blue-green wavelengths (490–560 nm), with water itself being one of the strongest contributors to overall attenuation. In contrast, the SB site was characterised by the highest and most variable values of Kd(PAR) (mean = 0.54 m−1), comparable to near-coastal waters, and the spring bloom started almost a month later than at the ND site. The vertical variability of the attenuation coefficient and the strong PAR attenuation in the blue region of the spectrum were the result of higher concentrations of phytoplankton, CDOM and SPM, due to riverine inputs, shallow depth and strong tidal mixing. The OG site showed intermediate conditions between the ND and SB sites with a mean Kd(PAR) = 0.23 m−1, and deepest penetration of irradiance in the green region of the spectrum at 560 nm. The implications of these results for phytoplankton growth and ecosystem modelling are discussed.

The mass–area relationship within cryoconite holes and its implications for primary production

AbstractLinear relationships between the mass of sediment present in a cryoconite hole and the hole area are described for a range of glacier and ice-sheet surfaces. The strong relationships found indicate that some mechanism regulates the thickness of the layer of sediment occupying the ‘floor’ of the hole. We find that this regulation process responds immediately to the addition of new debris to a hole and infer that it is caused by lateral thermal conduction from the debris to the hole wall. This causes hole widening by melt, and a redistribution of the debris within then takes place, usually resulting in 0.04–0.20 gcm−2 of debris in a layer of single cryoconite grains. The thinning of the debris layer during hole widening also reduces self-shading and thus maximizes the exposure of cryoconite to solar radiation. We explore the implications of the hole widening for biological production. Net photosynthesis (CO2 fixation) is shown to be favoured by thin debris layers, whilst net heterotrophy (CO2 respiration) occurs when debris layers are in excess of 2–4 mm. We conclude that the carbon balance of cryoconite holes is sensitive to the thickness of the debris and that the thermodynamic equilibration of the debris thickness helps the ecosystem to maximize primary production during the summer.

Inputs of glacially derived dissolved and colloidal iron to the coastal ocean and implications for primary productivity

Glacial meltwaters draining shield bedrock under the Greenland Ice Sheet (GIS) contain <0.4 μm “total dissolved” Fe (TDFe) with an average flow weighted concentration of ∼53 nM. The concentrations of <0.03 and 0.03–0.4 μm Fe vary over the ablation period, with weighted means for each of these fractions being respectively 22.4 nM and 30.8 nM. These concentrations are lower estimates as an adjacent larger glacier (a more representative source of glacial meltwater) had higher dissolved Fe concentrations, and reactions of meltwaters with proglacial sediments could also enhance dissolved Fe concentrations. This source of TDFe is additional to the reactive (oxyhydr)oxide phases identified by Raiswell et al. (2006) that are also introduced to adjacent polar seas from glaciers. The Fe concentrations in the shield bedrock underlying the GIS are lower than those of other crustal rocks (4.0% cf. 6.2%), but we argue that these Fe concentrations are not limiting on the total dissolved Fe concentrations we measure. The biogeochemical weathering processes operating on the subglacial debris and suspended sediment in our catchment are likely to be similar to those in other glaciated catchments. Therefore the meltwater Fe concentrations reported here can be used to give a first estimate of global fluxes of meltwater dissolved Fe to coastal polar waters. A lower estimate of the global flux of TDFe from glacial meltwaters is ∼75 × 106 moles Fe/a. This glacial meltwater input of Fe to adjacent polar waters will be greatest around Greenland where there are highest annual meltwater discharges. However, the greatest impact of this source of glacial meltwater Fe is anticipated to be in Antarctic high nutrient low chlorophyll (HNLC) waters where phytoplankton productivity is typically limited by availability of Fe. For Antarctic waters the estimated meltwater Fe (TDFe) input is about 10% of that suggested to come from sea ice melting, but glacial inputs continue throughout the austral summer ablation period after sea ice melt is complete.

External forcing by wind and turbid inflows on a deep glacial lake and implications for primary production

Lake Coleridge is a deep monomictic lake in the Southern Alps of New Zealand which is affected by strong adiabatic winds. It receives significant inputs of inorganic suspended solids from rivers diverted into the lake for hydro‐electric generation. External inputs of wind energy and suspended solids to Lake Coleridge were quantified throughout 1 year and their impacts on hydrodynamics and the light climate were analysed. During stratification, the lake had a deep mixed layer, underlain by a complex metalimnion. High wind energy inputs caused large internal waves which resulted in the upwelling of hypolimnetic water, horizontal density gradients, and significant currents. Calculated vertical diffusivity and exchange coefficients were similar to those of much larger lakes. Analyses indicated that mixing in the metalimnion was often a result of shear stresses arising from vertical gradients in horizontal velocities. Whole lake total suspended solids (TSS) burdens were significantly correlated with the previous 13‐week integrated TSS loading from inflows. A seasonal pattern was observed where TSS from inflows entered either below, within or above the metalimnion. Evidence of effects of sediment resuspension from the marginal lake bottom was observed only when the lake level was low and the resulting impacts on the light climate were minimal. The synergistic effects of climate, thermal structure, and inflows resulted in the apparent light‐limitation of phytoplankton photosynthesis in winter months.

The impact of phytoplankton on spectral water transparency in the Southern Ocean: implications for primary productivity

Spectral water transparency in the Northern Weddell Sea was studied during Austral spring. The depth of the 1-% surface irradiance level (“euphotic depth”) varied between 35 and 109 m and was strongly influenced by phytoplankton biomass. Secchi depths were non-linearly related to euphotic depth. In phytoplankton-poor water, the most penetrating spectral region was restricted to a relatively narrow waveband in the blue (∼488 nm), but the range was broader, between 488 and 525 nm when phytoplankton were abundant. Water transparency in the red spectral range was always low and only to a small extent affected by phytoplankton. Two independent procedures were used to quantify the impact of phytoplankton on spectral water transparency: (1) Regression analysis of spectral in situ vertical light attenuation coefficients in the sea, against coincident chlorophyll concentrations. This method gave chlorophyll-specific light attenuation coefficients; the y-intercept could be interpreted as a measure of light attenuation by pure water plus non-algal material. (2) Spectra of in vivo light absorption derived by spectroscopy, using phytoplankton enriched to varying degrees onto filters. Thus chlorophyll-specific absorption cross-sections were determined. Estimates obtained by both procedures were in close agreement. By integrating over the spectrum of underwater irradiance, in situ chlorophyll-specific absorption cross sections of phytoplankton suspensions, related to all photosynthetically active radiation, were calculated. Light absorption by phytoplankton for photosynthesis is accomplished mainly in the blue spectral range. Also dissolved and particulate organic matter contributed to the attenuation of blue light. Because in water poor in phytoplankton, underwater irradiance was progressively restricted to blue light, chlorophyll-specific absorption cross-sections of phytoplankton, averaged over the spectrum of photosynthetically active irradiance, increased with water depth. In water with elevated phytoplankton biomass, overall light attenuation was generally enhanced. However, because the spectral composition of underwater light changed relatively little with depth, except immediately below the water surface, light absorption cross-sections of phytoplankton changed little below 10 m depth. Vertical differences in the proportions of underwater light absorbed by the phytoplankton community here were mainly dependent on biomass variations. Because of the comparatively small attenuation of blue light by non-algal matter, the efficiency of light harvesting by phytoplankton at any given concentration of chlorophyll in Antractic waters is greater than in other marine regions. At the highest phytoplankton biomass observed by us, as much as 70% of underwater light was available for phytoplankton photosynthesis. When phytoplankton were scarce, <10% of underwater light was harvested by phytoplankton.

Mean transport of mass, heat, salt and nutrients in southern California coastal waters: implications for primary production and nutrient cycling

A close balance is demonstrated between geostrophic convergence, relative to 500 m, and Ekman divergence, in a control volume defined by the modern (1984–1987) CalCOFI surveys of southern California coastal waters. Both the geostrophic and Ekman calculations yield an area mean vertical velocity of about 3.5 × 10 −4 cm s −1 upward at the base of the Ekman layer. Cruise-to-cruise and year-to-year variations in geostrophic transport are large and a long time series is required for a good estimate of the mean field. With the mass field balanced by geostrophic and Ekman transport, the resulting oceanic heat flux is equivalent to 68 W m −2 of heat gain by the ocean and is in good agreement with calculations of air-sea heat transfer. Agreement in the heat budget sets a limit on possible errors due to ambiguity in the reference level of the geostrophic velocity field. Mean annual and seasonal fluxes of nutrients, oxygen, chlorophyll a, and salinity are calculated, together with error bars, and are used to estimate primary production as well as rates of air-sea oxygen exchange and net water vapor loss to the atmosphere. The nitrate import, converted to equivalent carbon by means of a Redfield ratio, and including local estimates of nitrogen recycling, yields an estimate of total primary production of roughly 600 mg C m −2 d −1, consistent with direct measurements of carbon uptake. The geostrophic convergence and subsequent upwelling of subsurface nitrate-rich waters is the dominant physical mechanism for nutrient importation, with a much smaller contribution due to alongshore advection of nutrients in the surface layer. It is suggested that some additional measurements would permit the flux calculations to be made with a substantially greater degree of accuracy and that the seasonal and interannual variations of the mass transport field could be determined together with rates of biologically and physically important processes.