Biogeochemical processes in Amazon shelf waters: chemical distributions and uptake rates of silicon, carbon and nitrogen

ContinentalShelf

Research. Vol. 16, No. 516, pp. 617-643, 1996

Pergamon

Copyright 0

1996 Elsevier Science Ltd

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Biogeochemical processes in Amazon shelf waters: chemical distributions and uptake rates of silicon, carbon and nitrogen D. J. DeMASTER,*

W. 0. SMITH JR,? J. Y. ALLER$

D. M. NELSONS

and

(Received 31 March 1994; accepted 1 June 1995)

Abstract-Biogeochemical processes in the Amazon River/ocean mixing zone were examined during four AmasSeds cruises between August 1989 and November 1991. On the Amazon shelf, the distributions of chlorophyll-a, oxygen supersaturation, pH and the biogenic-silica content of suspended matter all showed coherent patterns highlighting areas of high primary productivity. Phytoplankton blooms occurred seaward of the high-turbidity waters (suspended-solid concentration >lO mg I-‘) and the extent of the bloom depended on the supply of nitrate from the Amazon River, as well as the time available to phytoplankton in the “optimal-growth zone” (a region of relatively low turbidity and high nutrients). Of the four AmasSeds field cruises, the largest phytoplankton bloom occurred during March 1990, when the nitrate flux to the shelf was highest and the plume residence time was the longest. The biogenic-silica standing crop in Amazon shelf waters (area of 8.9 x 10”’ m*) varied from 29 to 66 x 10s moles during the two-year study period, whereas the depth-integrated silicate anomaly on the shelf (i.e. the difference between the observed nutrient concentration and the salinitypredicted value from the ideal mixing of riverine and oceanic end members) varied from +5 to -170 x 10” moles. Chlorophyll-a inventories on the shelf ranged from 10 to 36 x 10s g. The maximum extent of nutrient removal occurred during the March 1990 cruise, consistent with the maximum standing crops of biogenic silica and chlorophyll-a. Excess silicate and excess inorganicnitrogen concentrations (relative to the ideal mixing line) commonly occurred in inner-shelf waters (~20 m water depth) and were attributed to resuspension of nutrient-rich porewaters by tides and waves. If the standing crops of these excess nutrient distributions are divided by the residence time for inner-shelf waters (estimated to be -14 days), the resulting fluxes suggest that seabed resuspension provided on the order of 5-20% of the silicate and inorganic nitrogen reaching the shelf. A comparison of the magnitude and the locations of the silicate depletions with the biogenicsilica standing crops suggests that for most cruises a mechanism separating dissolved and particulate siliceous phases must be operating on the shelf. Aggregation, zooplankton grazing and/ or nutrient limitation (which increases the sinking rates of diatoms) are likely causes for the settling of the siliceous particles into the landward-moving bottom waters. Distributions of biogenic silica and organic carbon in surface suspended matter correlated poorly (linear regression R’ <0.06) during all sampling periods. The likely explanation for this observation is the occurrence of nonsiliceous phytoplankton and the grazing by zooplankton (which strip organic matter from the siliceous frustule). The Amazon shelf exhibited some of the highest carbon and silicon production rates in the marine realm. Rates of primary productivity during the AmasSeds cruises were as high as 250 mmol C mm3 day-.‘, whereas silicate uptake rates were as high as 15 mmol mW3 day-‘. Field

*Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, NC 27695-8208, U.S.A. tDepartment of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, U.S.A. fcollege of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, U.S.A. OMarine Sciences Research Center, State University of New York, Stony Brook, NY 11794-5000, U.S.A. 617

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measurements of carbon and silicon production rates were consistent with the changes in particulate organic-carbon and biogenic-silica concentrations that occurred along the trajectory of the plume during the phytoplankton’s 2-7 day transit through the “optimal-growth zone” of the shelf.

INTRODUCTION Nutrient cycling and associated biogeochemical processes have been studied in the river/ocean mixing zone on several of the world’s largest rivers, including the Amazon (Milliman and Boyle, 1975; Edmond et al., 1981; DeMaster et al., 1983, 1986), the Changjiang (Edmond et&., 1985; DeMaster and Nittrouer, 1983) and the Zaire rivers (van Bennekom et al., 1978). Large river systems tend to have several unique characteristics that distinguish them from the more numerous, smaller river systems of the world. In addition to the magnitude of the water discharge, large rivers commonly exhibit high turbidity and complex circulation patterns, as they mix with ocean waters (DeMaster etul., 1986). Understanding nutrient cycling in these complex estuarine regimes can be difficult, based solely on nutrient distributions. Rate measurements of nutrient uptake or carbon uptake can be very useful in unraveling the intricate biogeochemical cycling processes taking place in an estuary (e.g. Lohrenz et al., 1990; Nelson et al., 1994). Unfortunately, relatively few direct measurements of nutrient uptake rates have been made near the mouths of major river systems (Dortch et al., 1994). Another aspect of large river systems that often is not well characterized is the seasonal variation in nutrient dynamics and phytoplankton bloom development. Of the large rivers that have been studied (Amazon, Changjiang and Zaire) there is little or no information on seasonal variations in these biogeochemical processes and how they might be affected by temporal changes in river discharge, wind intensity, or coastal-current strength. By examining the same environment under several different oceanographic regimes, the factors controlling nutrient uptake and primary production can be resolved much more readily than from a study based upon a single period of field observation. This paper examines nutrient distributions, phytoplankton bloom development, organic-carbon and biogenic-silica production rates and zooplankton abundance on the Amazon shelf. The field data were collected as part of AmasSeds (A Multidisciplinary Amazon Shelf SEDiment Study), which is an interdisciplinary study of processes affecting particles on the Amazon shelf. An advantage of participating in an interdisciplinary project like AmasSeds is that in addition to the chemical analyses characterizing bloom dynamics, measurements of current speed, water residence time, winds and sedimenttransport rates were also made during the same time period. The combination of these investigations provides a powerful data set for characterizing the nature of nutrient uptake on the Amazon shelf, its variability in time, as well as the dominant oceanographic processes that control its magnitude. Measuring nutrient and carbon uptake rates complements the physical oceanographic data, because phytoplankton uptake rates can serve as biological clocks. These rates can be used to corroborate, or set limits on, environmental parameters such as surface water trajectories and residence times, as well as particle residence times. During the AmasSeds project, the Amazon shelf was sampled four times between August 1989 and November 1991, corresponding to different stages of river discharge (falling, rising, high and low discharge) as well as differing wind regimes and coastal-current intensity (Nittrouer and DeMaster, 1996). To resolve the dominant

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processes controlling primary production in the Amazon river/ocean mixing zone, the seasonal changes in bloom development were related to the temporal variations in river discharge, winds and coastal currents. The overall goal of this research was to examine the processes controlling nutrient uptake and primary production on the Amazon shelf. The specific objectives necessary to accomplish this goal were: (1) to examine the coherence of chemical tracers (e.g. biogenic silica, chlorophyll-a, oxygen, pH) during phytoplankton bloom development; (2) to characterize the changes in bloom coverage during the four AmasSeds field programs and examine environmental factors influencing bloom development; (3) to contrast dissolved silicate and inorganic-nitrogen anomalies in the water column with solid-phase, standing-crop inventories for all four cruises; and (4) to utilize organic-carbon and silica production rates along with particulate concentrations of these biogenic phases and surface-water residence times, to track the growth of phytoplankton blooms from a Lagrangian perspective. Although there are numerous types of phytoplankton on the Amazon shelf (Hulburt and Corwin, 1969) this study focuses primarily on diatoms, which are the predominant primary producers on the shelf and the biota most likely to be controlling the distributions of dissolved silicate and nitrate in the study area.

BACKGROUND Oceanographic

setting

Because of the immense discharge of the Amazon River, the river/ocean mixing zone occurs on the continental shelf. The mean discharge of the river is 1.8 x lo5 m3 s-l (Oltman, 1968) but during periods of high discharge (May/June) the fresh-water flow is twice as great as during low discharge (October/November). The dynamics of the plume region of the Amazon mixing zone were described several years ago by Curtin (1986a,b) and Curtin and Legeckis (1986), and more recently by Lentz and Limeburner (1995) and on the location on the shelf, as well as the tidal regime Geyer et al. (1996). Depending (high/low, spring/neap), the river/ocean mixing zone can be characterized by isohaline distributions consistent with the classic “salt wedge” model or the “partially well mixed” model of estuarine circulation (Geyer et al., 1996). Dissipation of tidal energy is a very important process on the shelf, affecting salinity distributions (Beardsley et al., 1995) as well as sediment transport (Kineke et al., 1996). Winds on the shelf are dominated by the easterly trade winds, which are most intense during January, February and March, and weakest during August and September (Picaut etal., 1985; Lentz, 1995). The North Brazil Current sweeps the outer-shelf with peak flows of 30 Sverdrups during August and a minimum flow of 10 Sverdrups during April (Philander and Pacanowski, 1986). The movement of surface waters on the shelf was monitored during AmasSeds using satellitetracked drogues (Limeburner et al., 1995) and by using the ADCP data from the individual cruises (Candela et al., 1992). After removing the tidal accelerations, the mean velocities of the surface-plume waters on the shelf are on the order of 0.3-l m s-l (Limeburner eral., 1995). The sediments shoreward of the 60-m isobath are composed primarily of silt- and clay-size material (Nittrouer and DeMaster, 1996) with fluid muds (Kineke et al., 1996) commonly l-2 m thick, overlying much of the inner-shelf (water depths ~20 m).

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Suspended matter from the Amazon River goes through cycles of trapping and resuspension near the river mouth, prior to along-shelf transport and incorporation into fluid-mud mechanisms for resuspension of inner-shelf layers (Kineke et al., 1996). The dominant sediments are tidal processes (Beardsley et al., 1995) and wave activity (Sternberg et al., 1996).

Previous

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Ryther et al. (1967) and Hulburt and Corwin (1969) were among the first to examine nutrient and phytoplankton systematics near the mouth of the Amazon River. These authors suggested that the riverine nutrient supply is relatively small and that most of the bloom occurring on the outer-shelf is sustained by upwelling of nitrate- and phosphate-rich water. Milliman and Boyle (1975) recognized the important effect of turbidity on the biological uptake of dissolved silicate on the shelf. Based on distributions of nitrate, silicate, phosphate and oxygen from a 1976 May/June cruise to the Amazon shelf, Edmond et al. (1981) concluded that despite extensive uptake of nutrients in surface waters, carbon and phosphate remineralization was nearly complete within the river/ocean mixing zone. They also concluded that half of the removed nitrate was “solubilised to species other than nitrate and nitrite” and that only 20% of the silicate removed from shelf surface waters could be accounted for in the salt wedge below. DeMaster et al. (1983, 1986) measured dissolved silicate in Amazon shelf waters and biogenic silica in shelf sediments collected during cruises in October 1979 and in May 1983. They reported that uptake in surface waters accounted for a third to half of the dissolved silicate coming down the river, but burial of biogenic silica in the Amazon delta corresponded to less than 4% of the riverine silicate flux. Therefore, nearly all of the silicate coming down the river ultimately makes it to the open ocean. The occurrence of clay-mineral formation on the Amazon shelf has been suggested recently (Aller, personal communication); however, the impact of this process on silica budgets is probably small. Riverine supply, uptake by diatoms and dissolution of biogenic silica in the water column in Amazon shelf and seabed are the dominant flux terms. Based on “(‘Pb inventories sediments, DeMaster and Pope (1996) and Smoak et al. (in press) estimated the flux of offshore water that must move up and onto the shelf during the estuarine mixing process to be -1 8 x 10” m3 s-’ (or 5.6 x lOI I y-l). Using the 210Pb-derived flow rate and isopycnal distributions (to establish the depth of the source water) DeMaster and Pope (1996) concluded that most of the silicate and nitrate supporting outer-shelf phytoplankton blooms came from the river, whereas most of the phosphate and ammonium came from shoreward advection of offshore waters. Burial rates of organic matter on the shelf suggest that only 15% of the nitrate and phosphate released during river/ocean mixing is accumulating in the deltaic sediments of the shelf (DeMaster and Pope, 1996). Aller et al. (1996) estimate that at least 25% of the organic carbon produced in the water column is regenerated within the seabed. There have been very few research programs aimed at quantifying rates of primary production on the Amazon shelf. Teixeira and Gaeta (1991) used the 14C-uptake method to examine the importance of picoplankton productivity in the high-salinity shelf waters, seaward of the Amazon River mouth. They found that picoplankton (size fraction 0.451 .O pm) were responsible for 7-100% of the productivity in high-salinity (>36 PSU) were used to offshore waters. During the AmasSeds cruises, 14C incubation measurements

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quantify rates of primary production, which were highest (in some places exceeding 3000 mg C rnp3 day-‘) in the low-turbidity waters northwest of the river mouth (Smith and DeMaster, 1996). Silica production rates and dissolution rates from the AmasSeds Project plume reveal net uptake values as high as 15 mmol rnp3 day-’ seaward of the high-turbidity (DeMaster et al., 1991, 1995; Nelson, 1992).

METHODS Field techniques Nutrients (silicate, phosphate, nitrate, nitrite, ammonium and urea), oxygen and pH were measured at approximately 55 stations during each of the four AmasSeds cruises (August 1989 -falling river discharge; March, 1990 -rising river discharge; May 1990high river discharge; and November 1991low river discharge). Water and suspendedparticulate samples were collected at approximately six depths per station, using a rosette configured with 10-l Niskin bottles. Salinity was measured using a Neil Brown Instrument System (MKIII CTD, equipped with a 5-cm pathlength Sea Tech transmissometer and a Sea Tech fluorometer) and a SEABIRD SEACAT CTD (equipped with an oxygen sensor and a pH electrode). Measurements on suspended material consisted of concentration, biogenic-silica content and organic-carbon/nitrogen content. Water samples for biogenicsilica analyses were filtered using Nuclepore filters (0.4 micron nominal pore size, 47 mm diameter), whereas organic-carbon and organic-nitrogen samples were filtered using precombusted Whatman GF/F filters (nominal pore size 0.7 micron, 25 mm diameter). After passage through the Nuclepore filters, water samples for nutrient measurements (silicate, nitrate, nitrite, ammonium, phosphate and urea) were analyzed using a Technicon II Autoanalyzer [see DeMaster and Pope (1996) for data and specific analytical procedures]. Based on duplicate analyses during the field program, the precision of the nutrient measurements was on the order of 2-5%. The oxygen-electrode output was calibrated at every other station, using a Winkler titration measurement of surface and bottom waters (precision = l-2%). The pH sensor on the SEABIRD CTD was calibrated at every station using a Fischer pH meter (model 955) and a pH 8 standard (NBS-based). Analytical procedures for measuring chlorophyll-a concentrations and 14C primary production rates in Amazon shelf waters are described in Smith and DeMaster (1996). Rate measurements for silicate uptake were made during the AmasSeds cruises using the silicon stable-isotope technique described in Nelson and Goering (1977). Zooplankton samples were sorted from total plankton assemblages, which were collected from surface waters using net tows (1 .O m diameter opening, 150 micron mesh net, 62 micron mesh in cod end) during each of the AmasSeds cruises. The volume of water passing through the net was measured by a rotor-type current meter (from General Oceanics) in the opening of the plankton net. The net tows were made on a different leg than the one that generated all of the other hydrographic measurements cited. During Cruise I, the hydrographic leg immediately preceded the plankton-collection leg (mean difference in sampling -12 days) whereas during Cruises II, III and IV, the hydrographic leg followed the plankton-collection leg (mean differences in sampling times were: 13, 12 and 21 days, respectively). As the trends and relationships examined in this study were primarily seasonal in nature, these relatively short offsets in sampling time (as well as daily variations) should not affect the results significantly. For comparative purposes, zoo

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plankton samples on AmasSeds cruises were collected from a nearshore (
Laboratory procedures Biogenic-silica content was measured using a modification of the technique described by DeMaster (1979, 1991). Owing to the large number of samples (-300 per cruise) the silicate extracted by the 1% NazCOs solution was measured only after 2 h of leaching (not the 5-h time-series approach). Under these leaching conditions, all of the biogenic silica in the suspended-sediment sample should be extracted, but the alkaline solution may also leach minor amounts of dissolved silicate from co-existing clay minerals. The amount originating from aluminosilicates, however, is believed to be -1 wt%, based on measurements of turbid-plume suspended-sediment samples from AmasSeds and on previous double-extraction time-series measurements conducted on sediments from the field area (DeMaster et al., 1983). Consequently, 1% was subtracted from the total silica extracted after 2 h to obtain the biogenic-silica content of the water sample. This correction must be made, or filters laden with high amounts of lithogenic material will appear to contain large amounts of biogenic silica (on a pmol 1-i basis). The precision of the biogenic-silica measurement is -5%. Total-carbon measurements were made using a Carlo Erba 1108 CNS analyzer. Approximately 6-10 filters were chosen from each cruise to evaluate the presence of inorganic carbon. Based on microscopic inspection of the filters and dilute acid treatment (1 M HCl), it was concluded that inorganic carbon was an insignificant contributor to the total-carbon signal. Therefore, the results from all of the samples were reported as organic-carbon content.

RESULTS

AND

DISCUSSION

Chemical parameters indicating bloom coverage During AmasSeds Cruise I (August 1989) the distributions of the four chemical tracers (surface water pH, chlorophyll-a, oxygen saturation and biogenic-silica content of suspended particles) highlighted consistent areas of high biological activity (Fig. 1). Areas characterized by phytoplankton blooms generally exhibited biogenic-silica contents >lO wt% for surface suspended matter (indicating abundant diatoms) as well as substantial oxygen supersaturation (consistent with excess photosynthetic activity relative to respiration). In bloom areas, pH values were abnormally high as a result of the consumption of carbon dioxide during photosynthesis (equivalent to removing carbonic acid). Lastly, blooms were readily apparent from chlorophyll-a distributions, because the predominant species of phytoplankton in most blooms are diatoms (Hulburt and Corwin, 1969; Knapp, 1981), which commonly contain high levels of this pigment. On a shelf-wide basis, the coherence among these four chemical tracers was relatively low [linear-regression coefficient of determination (R*) = 0.2-0.51, presumably because each of them responds differently to the river/ocean mixing processes and the gradients in suspended-solid

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Fig. 1. Distributions of four chemical tracers indicating areas of phytoplankton blooms during AmasSeds Cruise I (August, 1989): (a) biogenic-silica content of surface suspended matter (SSC = suspended-solid concentration); (b) % oxygen saturation in surface waters; (c) chlorophyll-a content of surface waters; and (d) pH of surface waters. Although the mixing of river and ocean waters affects the distribution of these tracers in different ways, all four of them show a coherent signal where there is abundant phytoplankton production (i.e. the northwest corner of the study area).

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Fig. 2. Distribution of high turbidity (suspended-solid concentration or SSC >I0 mg I-‘) and highly siliceous suspended matter (>lO wt%) in Amazon shelf surface waters during the four AmasSeds cruises. There was little overlap in these regions, because high turbidity limits phytoplankton growth. The waters containing highly siliceous material indicate areas of diatom blooms. The area1 coverage of the bloom was largest during Cruise I1 (March 1990) and smallest during Cruise IV (November 1991).

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Fig. 4. Distribution of water-column biogenic-silica inventories on the Amazon shelf. During the first three cruises, the diatom blooms primarily occurred on the outer-shelf, whereas during Cruise IV (a period of low river discharge) the diatoms occurred primarily in mid-shelf waters.

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of the depth-integrated silicate anomaly in Amazon shelf waters. The Fig. 5. Distribution anomaly was calculated by subtracting the silicate concentration predicted from the ideal mixing line (between riverinc and open-ocean waters) from the observed silicate concentration. Negative values indicate arcas of net nutrient uptake. whereas positive values indicate areas of net nutrient release. The area just northwest of the river mouth typically is characterized by excess nutrients, probably as a result of tidal resuspension and wave activity. The largest silicate depletion on the shelf occurred during Cruise II (March l990), when the nitrate flux to the shelf was the largest, the fresh-water residence time was the longest and the bloom devclopmcnt was the greatest.

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Fig. 6. Distribution of depth-integrated inorganic-nitrogen anomaly. The anomaly was calculated by subtracting the inorganic-nitrogen concentrations (nitrate, nitrite and ammonium) predicted from the ideal mixing line of riverine and oceanic end members from the observed inorganic-nitrogen concentrations. The anomaly values for nitrate, nitrite and ammonium were summed to produce the inorganic-nitrogen anomaly. Excess inorganic nitrogen (i.e. positive values) commonly occurred on the inner-shelf north of the river mouth, whereas the inorganicnitrogen depletions (i.e. negative values) typically occurred on the outer-shelf.

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concentration. In areas of significant bloom development, however, these subtle variations are overwhelmed by the chemical signatures generated by the growth of marine phytoplankton and coherence in the tracer signals is observed (Fig. 1). Primary production was measured during Cruise I using the 14C-uptake method, but the number of stations (n = 12) was limited and the distribution pattern of the bloom was not as apparent as with the other biogeochemical indicators. To a first approximation, the areas of high primary productivity (2 3 g C m-’ day-‘; Smith and DeMaster, 1996) coincided with the areas of abundant bloom occurrence. Therefore, the various tracers of bloom development (biogenic silica, oxygen, pH, chlorophyll-a) and primary productivity reveal a fairly coherent signal during Cruise I, with the greatest bloom coverage and highest productivity occurring in the northwest portion of the study area. Temporal variability in bloom coverage The distribution of biogenic-silica content in surface suspended matter can be used to identify areas of bloom development during the four AmasSeds cruises (Fig. 2). As described in the previous section, bloom occurrence typically is characterized by biogenicsilica contents in excess of 10 wt% , which corresponds to suspended-solid concentrations on the order of l-10 mg 1-i (or approximately 8pmol Si 1-l). The dominance of siliceous biota (i.e. diatoms) in these blooms is evident from the high molar ratio of biogenic silica to organic carbon observed in bloom material (typically 0.2-1.3). Bloom coverage was the most extensive during Cruise II (rising river discharge). Cruises III and IV (high river discharge and low river discharge, respectively) exhibited the smallest area1 coverage of silica-rich seston. In general, the areas showing high biogenic-silica abundance overlapped very little with the areas of high turbidity (>lO mg 1-l) as observed in previous studies (Milliman and Boyle, 1975; DeMaster et al., 1986). One potential cause for the variability in bloom development is the riverine supply of nitrate to the shelf. DeMaster and Pope (1996) showed that the dominant source of nitrate to Amazon shelf phytoplankton blooms is from the Amazon River, which supplies 2-3 times more of this nutrient than the landward advection of offshore water. Although the largest flux of fresh water during the field study occurred during the period of high river discharge (Nittrouer and DeMaster, 1996), the largest riverine flux of nitrate to the shelf occurred during rising river discharge, when the nitrate concentration in the river was unusually high (23 pmol kg-’ vs an annual mean of 16pmol kg-‘). For the four AmasSeds cruises, a linear regression of riverine nitrate flux vs the shelf-wide, water-column integrated, standing crop of biogenic silica [Fig. 3(a)] yields a coefficient of determination (R2) of 0.91 (p = 0.048). The standing crop of biogenic silica on the shelf was used as an indicator of biological production because of the sampling density (-55 stations per cruise) and the time-integrated nature of this chemical signal (see discussion below and silicon and carbon production-rate section). A second factor affecting the development of phytoplankton blooms on the Amazon shelf is the residence time of surface waters. Despite the high abundance of nutrients (e.g. nitrate, silicate, phosphate) in river water, phytoplankton can utilize these nutrients only over a relatively small area of the shelf because of the effects of turbidity (i.e. light limitation; Milliman and Boyle, 1975; DeMaster et al., 1986) and dilution by nutrient-poor offshore waters. If the transit time between the light-limiting edge of the turbidity front (-10 mg 1-l) and the dilution with offshore waters (salinity >33 psu) is short relative to the

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doubling time for phytoplankton growth, bloom development will be restricted. The longer the phytoplankton can remain in the zone of optimal growth (i.e. low turbidity and high-nutrient/low-salinity water), the greater the chances for extensive biological production and bloom occurrence (presuming that grazing effects are minimal). A linear

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Fig. 3. (a) Plot of riverine nitrate flux vs biogcnic-silica standing crop for the Amazon shelf during the AmasScds Cruises I. II. III and IV. The high cocfficicnt of determination (linear rcgrcssion R’ = 0.91) suggests that the riverinc nitrate flux plays an important role in governing phytoplankton bloom development. (b) Plot of fresh-water rcsidcnce time on the shelf vs hiogenicsilica standing crop for each of the four AmasSeds cruises. The greater the time the phytoplankton spend on the shelf and in the optimal-growth zone (low turbidity and high nutrients). the greater the magnitude of the bloom.

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regression of fresh-water residence time on the shelf vs the biogenic-silica standing crop [again used as an index of integrated biological production, Fig. 3(b)] yields a coefficient of determination (R’) of 0.58 (p = 0.25). The fresh-water residence times for the four cruises were calculated by integrating the total amount of fresh water on the shelf and then dividing the value by the fresh-water river discharge for that time period (Limeburner, personal communication). In this way, the residence times for the shelf waters were integrated over space and time, providing a signal commensurate with that of phytoplankton growth and bloom development (a time scale of days to a few weeks). One of the dominant processes affecting the residence time of surface waters on the shelf is the direction and intensity of the trade winds (Len& 1995). If the trade winds have a component in the direction of the plume (i.e. to the northwest) the residence time of the surface waters in the optimal-growth zone will be diminished, because the wind enhances the offshore transport and mixing between low-salinity, nutrient-rich waters and highsalinity, nutrient-poor waters. If the winds, however, have a component counteracting the plume flow, the waters will be held on the shelf longer, allowing increased time for phytoplankton growth. As described above, the largest phytoplankton-bloom development occurred during Cruise II (March, 1990). Just prior to this cruise, a period of strong trade winds occurred (Lentz, 1995) with a significant component opposing the plume flow (see Geyer et al., 1996). This wind caused the fresh water from the river to be retained on the shelf, increasing the residence time of shelf waters and creating an environment conducive to phytoplankton growth. Because of these strong winds during late February, the standing crop of fresh water on the shelf was largest during Cruise II, despite the fact that the river discharge was 24% less during Cruise II than during Cruise III (the period of high river discharge). Increased residence time plus the enhanced nitrate flux during Cruise II (see above) were probably key factors contributing to the large bloom observed during this time. Although the AmasSeds cruise schedule was designed to examine seasonal variations in Amazon shelf processes, bloom development may be influenced substantially by physical factors varying on shorter time scales such as tides (semi-diurnal and fortnightly oscillations), winds and cloud cover/solar irradiation. Some of these shorter-term variations are integrated to some degree in the data sets, because the field sampling (for all four of the cruises) generally occurred over a period of approximately 7-10 days. Temporal variations in biogenic-silica and nutrient standing crops The standing crop of biogenic silica in the water column was calculated for each station on all four cruises (Fig. 4) The biogenic-silica content of suspended solids was multiplied by the suspended-solid concentration, divided by the molecular weight of silica and integrated with depth in the water column (typically five or six data points) to produce the data contoured in Fig. 4. The contour intervals were integrated areally to yield the total biogenic-silica standing crop for each cruise (Table 1). The area of the shelf that was integrated for this calculation (Fig. 4) extended from about the S-m isobath out to the 100m isobath and from the Para River transect northwestward to the Cabo Orange transect approximately (8.9 X 10” m2). The dissolved-silicate anomaly and the inorganic-nitrogen anomaly also were calculated for each station on all of the cruises. These anomalies were calculated by subtracting the salinity-predicted nutrient value (based on the nutrient/salinity plot and the ideal mixing

632

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et al.

line connecting the riverine and oceanic end members) from the observed nutrient concentration. The oceanic end member used in these calculations was offshore, subsurface water (typically from a depth of 60-100 m) which, based on isopycnal surfaces, typified the composition of much of the ocean water moving landward (DeMaster and Pope, 1996). In the case of inorganic nitrogen, the anomaly values (observed minus the predicted) for nitrate, nitrite and ammonium were summed (molecular nitrogen was not measured). These anomaly data were integrated vertically over the five or six depths sampled for nutrients. If nutrient uptake was greater than regeneration, the anomaly was negative (Figs 5 and 6). Conversely, if the regenerated nutrient signal exceeded uptake in the overlying waters, the value was positive. As with the biogenic-silica data, the nutrientanomaly values from individual stations on a specific cruise were contoured and then areally integrated to obtain the total anomaly value for the entire Amazon shelf during that sampling period [Table 1; see DeMaster and Pope (1996) for data from a typical cross-shelf transect]. Comparing the distributions of the biogenic-silica inventory, the silicate anomaly and the inorganic-nitrogen anomaly, provides useful insights into nutrient cycling on the shelf. During Cruise I (falling river discharge) much of the inner-shelf (<20 m water depth) north of the Amazon River mouth exhibited excess silicate (Fig. 5) and to a lesser extent, excess inorganic nitrogen. The source of the excess nutrients was probably the seabed, which can be reworked by strong tidal currents and wave activity in this area (Beardsley et al., 1995; Kineke et al., 1996; Jaeger and Nittrouer, 1995; Kuehl et al., 1996). Aller et al. (1996) and DeMaster and Pope (1996) showed that for most Amazon shelf sediments, the diffusive flux out of the seabed is minimal for silicate and inorganic nitrogen (not including molecular nitrogen). Therefore, the resuspension of surface sediment by tides and waves may be the primary mode by which nutrients from the seabed are cycled back into the water column. The magnitude of this recycled flux may be approximated by integrating the excess anomaly data areally over the inner-shelf and then dividing the value by the average residence time of the inner-shelf water column. This approach yields a maximum estimate of benthic flux because it assumes that all of the excess nutrient signal comes from the seabed (no water-column regeneration in this area). It also assumes that the observed excess-nutrient standing crops are typical for the area and time period. A 14-day residence time was chosen for the inner-shelf, because it approximates the lowest values for fresh-

Table 1.

Biogenic

Cruise

Date

I

8/89 3190 5/90 11191

II 111 IV

standing

crops and inventories. nutrient anomalies AmasSeds cruises

Biogenic-silica standing crop” 35 66 35 29

Silicate anomaly* -19 -170 +5 -32

Inorganic nitrogen anomaly’ -3s -so +14 ~ 10

and zooplankton

Chlorophyll-a inventory-l-

abundances

Phaeophytin inventoryt

10 36 3.5 13

*Units for all standing-crop and anomaly values arc X10” moles. For the anomaly indicate nutrient depletion, whereas positive values indicate nutrient excess. -FUnits for chlorophyll-a and phaeophytin inventories arc: x 10’ g. *Units for the mean zooplankton abundance (six stations) = individuals per 100 ml.

5.3 42 IS 6.4 data,

during

Zooplankton abundancei X10 400 660 2220 negative

values

Biogeochemical processes in Amazon shelf waters

633

water residence time on the inner-shelf as well as the frequency of the spring/neap tidal cycle, which is probably a major contributor to the nutrient release. Based on these assumptions, the excess silicate anomaly (18 x lo8 moles) and the excess nitrogen anomaly (6.7 x lo* moles) on the inner-shelf (during Cruise I) were used to generate maximum fluxes of silicate (1500 moles s-l) and inorganic nitrogen (550 moles s-l) from the seabed, which corresponded to a relatively small fraction (~5% and <12%, respectively) of the total silicate and total inorganic nitrogen supplied to the shelf (primarily via the Amazon River and upwelling; DeMaster and Pope, 1996). The residence times of surface drogues in these inner-shelf waters were on the order of 5 days (Limeburner etal., 1995). However, most of the excess nutrient signal occurs in bottom waters, which are believed to have a substantially longer residence time than those at the surface. Even if the shorter residence time was used, the fluxes of silicate and inorganic nitrogen from the seabed never exceeded 12% and 27% (respectively) of the total silicate flux and total inorganic-nitrogen flux to the shelf during Cruise I. South of the Amazon River mouth, there is less tidal-current reworking than to the north and no detectable excess nutrient signal was observed in the water column during Cruise I. Near the mouth of the Para River, there was a substantial depletion in silicate during this period without a corresponding signal for biogenic silica. This observation may be explained, at least in part, by the local effects of the Para River, which has a substantially lower silicate concentration (70pmol kgg’; DeMaster and Pope, 1996) than that of the Amazon River (145 pmol kg-‘). A potential source of silicate to the shelf is the dissolution of fresh-water diatoms entering the mixing zone from the Amazon and Para Rivers. The importance of this source is believed to be relatively minor, based on the limited number of fresh-water diatom frustules observed in suspended-sediment samples from these rivers (Knapp, 1981). Near the northwest corner of the study area, the outershelf exhibited a net excess silicate signal. This occurred, despite silicate uptake in surface waters, because the water-column silicate anomaly was integrated to a depth of 100 m and the silicate-rich subsurface waters from the equatorial Atlantic were upwelling onto the shelf (DeMaster and Pope, 1996). During Cruise II (rising river discharge) biogenic-silica-rich waters covered a larger area than during any of the other cruises (Fig. 4). Similarly, the waters characterized by a negative silicate anomaly (Fig. 5) and negative inorganic-nitrogen anomaly (Fig. 6) also were the most laterally extensive of any of the field cruises. The locations of the silicatedeficient and the inorganic-nitrogen-deficient waters were fairly coherent [linear regression yielding a coefficient of determination (R2) = 0.531 but the locations exhibiting silicate depletion did not coincide with the areas showing the highest biogenic-silica standing crop (R2 = 0.02). The silicate removal was most prevalent in the outer-shelf waters, whereas the abundance of biogenic silica was greatest in the mid-shelf region. Therefore, a mechanism must decouple the dissolved and particulate siliceous phases during this period (see later discussion). The inner-shelf waters north of the river mouth still exhibited excess silicate values during this period and a rather localized excess inorganic-nitrogen signal. The combination of excess silicate on the inner-shelf and extensive silicate uptake on the outer-shelf created a sharp gradient in the silicate anomaly across the mid-shelf, with most of the isopleths running parallel to the bathymetry (Fig. 5). During high river discharge (Cruise III) extensive areas on the inner-shelf exhibited excess silicate and excess inorganic nitrogen (i.e. positive anomalies) indicating that this period (May 1990) may have been a time of enhanced porewater/water-column exchange

634

D. .I. DeMaster et 01.

(Figs 5 and 6). The locations of these excess signals were consistent with the areas of high tidal-energy dissipation (Beardsley et al., 1995), which suggests that tidal resuspension may be an important mechanism for porewater/water-column exchange. For the innershelf, both the areally integrated excess-silicate anomaly (39 x 10s moles) and the integrated inorganic-nitrogen anomaly (12 x lo* moles) were twice as large during Cruise III as during Cruise I. Based on a 14-day residence time estimate for inner-shelf waters (and the assumptions described above) resuspension of bottom sediments released as much as 10% of the silicate supplied to the shelf and as much as 20% of the inorganic nitrogen (nitrate, nitrite and ammonium) reaching the shelf during this time period. If the residence time of the inner-shelf waters was as short as 5 days, the flux of inorganic nitrogen from seabed resuspension during Cruise III could have been comparable to that supplied by riverine and upwelling sources. Consistent with the high turbidity and lack of silicate uptake in nearshore waters. the inner-shelf was nearly devoid of biogenic silica during Cruise III (Fig. 4) except near the mouth of the Para River. Complementing the nutrient data are the 224Ra measurements of Moore et al. (in press) and Moore et al. (1996). This short-lived radium isotope (ri12 = 3.6 days) accumulates in Amazon shelf porewaters from the decay of 228Th and consequently can serve as an independent tracer of porewater nutrients. Moore et al. (in press) reported that during Cruise III, the amount of 224Ra on the shelf was nearly twice as high as during any of the other cruises. During low river discharge (Cruise IV) the inner-shelf waters showed evidence of nutrient enrichment at and north of the river mouth (Figs 5 and 6). This pattern is the same as in the other three cruises. The outer-shelf waters north of the river mouth exhibited excess inorganic nitrogen which, as discussed earlier, probably resulted from nutrient-rich waters moving up onto the shelf at depth. Most of the biogenic-silica standing crop during Cruise IV occurred in the inner- and mid-shelf regions (Fig. 4), whereas most of the silicate depletion (negative silicate anomaly) occurred in the offshore waters northwest of the river mouth (Fig. 5). The location of highly siliceous material (> 10 wt%) in surface waters was confined to a relatively small area (Fig. 2) compared to that covered by the high biogenic-silica standing crops integrated over the entire water column (Fig. 4). The reason for this difference is that the mid-shelf waters had high biogenic-silica contents in their mid-depth and bottom waters, but very little silica in their surface waters. These stations with high biogenic-silica inventories, however, did exhibit silicate depletion in surface waters, even though the surface suspended matter at the time of collection did not contain significant amounts of the siliceous phase. As in the results from Cruise II, these data suggest that the dissolved and particulate phases for silica may be decoupled to some degree within the study area. Although the differences in the locations of extensive silicate uptake and high biogenic-silica inventory were most readily apparent in Cruises II and IV, the mechanism causing the decoupling of the dissolved and particulate phases probably occurs commonly on the shelf. The linear regression between the silicate anomaly (inpmol kg--‘) and the biogenic-silica abundance (in pmol kg-‘) yielded a very low coefficient of determination (R2 < 0.06) for all four cruises. During Cruise IV, the silicate anomaly had better coherence with the inorganic-nitrogen anomaly (linear regression R2 values ranged from 0.33 to 0.44) than with biogenic-silica abundance, but the correlation was still relatively weak (see below for discussion). Lohrenz et al. (1990) correlated silicate and dissolved inorganic-nitrogen concentrations in the Mississippi plume and observed an R2 value of comparable magnitude (-0.37).

Biogeochemical

processes

Shelf-wide comparisons of biogeochemical

in Amazon

shelf waters

635

parameters during AmasSeds

Table 1 shows the values for biogenic silica, the silicate anomaly and the inorganicnitrogen anomaly integrated spatially over the entire shelf for each of the AmasSeds cruises. The table also provides the water-column inventories of chlorophyll-a and phaeophytin (Smith and DeMaster, 1996) as well as the zooplankton abundances for the four cruises. In agreement with the previous discussion, Cruise II data showed the largest biogenic-silica standing crop and the largest nutrient depletions. The standing crop of chlorophyll-a was greatest during Cruises II and III. The phaeophytin concentrations, a semi-quantitative indicator of the amount of chlorophyll that has been processed by zooplankton (Lorenzen, 1967); however, were significantly higher during Cruise II than during Cruise III. By adding the chlorophyll-a concentration to the phaeophytin value, a pigment number can be obtained that reflects total bloom production and is less sensitive to the extent of zooplankton grazing. For the four cruises, the linear regression between the biogenic-silica standing crop and the chlorophyll-a inventory has a coefficient of determination (R’) of only 0.41 (n = 4, p = 0.36), whereas the correlation between the biogenic-silica standing crop and the summed pigment value (chlorophyll-a plus phaeophytin) has an R2 value of 0.76 (n = 4,p = 0.13). These data are consistent with the results of Cowie and Hedges (1995) who observed that zooplankton grazing could kill viable diatoms, but the associated biogenic silica did not necessarily dissolve in the process. Surprisingly, only Cruise IV data showed any kind of balance between silicate removal and the biogenic-silica standing crop. During Cruise I and Cruise III, there was more biogenic silica in the water column than there was silicate removal. This cannot be sustained on a steady-state basis, but may occur during periods when tidal resuspension of nutrient-rich porewaters is comparable to, or exceeds, the net nutrient uptake from phytoplankton activity. Another way that the biogenic-silica standing crop could exceed silicate depletion on the shelf is for the siliceous particles to be preferentially retained on the shelf, relative to the nutrient-depleted water from which they were formed. If siliceous particles sink in the water column as a result of aggregation with other material (Sancetta et al., 1991) or grazing followed by defecation, the siliceous material will be retained on the shelf (because of the estuarine circulation moving subsurface waters landward) and will be decoupled from the nutrient-depleted water moving out of the shelf area in the surface plume. Another related mechanism for separating dissolved and particulate siliceous phases has been described by Bienfang et al. (1982) and Smetacek (1985) who noted that nutrient depletion in marine diatoms can cause enhanced sinking rates (as a result of increased particle densities) by a factor as high as 10. Therefore, if diatoms on the Amazon shelf become silicate limited as they are mixed with the nutrient-depleted offshore waters, they may sink and be carried landward by the estuarine subsurface flow. This mechanism is consistent with the data in Cruises II and IV, when the high biogenic-silica standing crops occurred nearer shore and low nutrient concentrations occurred in outer-shelf waters. The landward transport of biogenic silica in subsurface waters supports the observation of relatively uniform rates of organic-carbon regeneration in shelf sediments, which requires the landward transport of organic-rich particles from outer-shelf waters (Aller et al., 1996). During Cruise II, the negative silicate anomaly exceeded the biogenic-silica standing crop. This is the more typical marine situation (Nelson and Smith, 1986) and commonly is explained by deposition of siliceous material on the seabed as enhanced by aggregation,

636

D. J. DeMaster

ef al.

grazing, or nutrient depletion. DeMaster et al. (1986) also noted extensive silicate depletion seaward of the Amazon River mouth during a May 1983 cruise and, similarly, little biogenic silica remained in the water column. There is no evidence of abundant biogenic silica occurring in Amazon shelf sediments (DeMaster et al., 1983). However, on a short-term basis, some of the siliceous material from the euphotic zone may end up in the fluid muds and surface sediments of the shelf, where ultimately it may dissolve, releasing the silicate back to the water column during times of intense tidal resuspension or wave activity. Therefore, the differences between the integrated silicate uptake and the biogenic-silica standing crops during three of the four cruises require mechanisms for removing siliceous particles from the waters in which they originated. The linear regression between the integrated silicate removal and the integrated inorganic-nitrogen removal for the four AmasSeds cruises has a coefficient of determination (R2) of 0.61 (PZ= 4, P = 0.22). If diatoms were the only plankton removing nitrate on the shelf, a higher regression coefficient would be expected between silicate removal and inorganic-nitrogen removal, because of the diatoms’ nutritional needs. However, not all of the phytoplankton on the shelf are diatoms (Hulburt and Corwin, 1969). Another process decoupling the silicate and inorganic-nitrogen depletions is zooplankton grazing. As described above, some of the organic-matter nitrogen ingested by zooplankton may be recycled back to the water column in dissolved inorganic form (such as ammonium), whereas the biogenic silica may be released having undergone only minor dissolution. A plot of biogenic-silica content vs organic-carbon content for surface suspended material corroborates these ideas (Fig. 7). For the Cruise IV data, the linear regression between biogenic-silica content and organic-carbon content shows no significant relationship (R2 < 0.06) primarily as a result of the two rather independent populations: one with high organic-carbon and low biogenic-silica contents (20% organic carbon, O-5% Si02) and the other very little organic-carbon but high biogenic-silica contents (l-5% organic carbon,

Org.

0

10

Wt.

%

C/Silica

= 5

20

Biogenic-Silica

30

40

Content

Fig. 7. Wt% organic-carbon content vs wt% biogenic-silica content for surface suspended matter during AmasSeds Cruise IV. The lack of coherence (coefficient of dctcrmination 10.06) between these biogenic phases is attributed to differing assemblages. The high organic carbon/low silica points probably represent non-siliceous plankton on the shelf, whereas the low organic carbon/high silica points probably result from diatoms subjected to grazing by zooplankton.

Biogeochemical processes in Amazon shelf waters

637

20-30% Si02). This lack of correlation between organic carbon and biogenic silica is typical of all the AmasSeds cruises. The populations with high organic carbon and low silica probably have a considerable fraction of non-siliceous phytoplankton in them, whereas the populations with low organic carbon and high silica (organic carbon to silica wt ratio of -0.15) probably contain substantial amounts of biogenic silica that have been grazed by zooplankton. For comparison, the organic carbon to silica ratio (on a wt% basis) for most tropical marine diatoms is -1-l 5 (Brzezinski, 1985). Cowie and Hedges (1995) showed that copepod feeding (Calanus pacificus) on diatoms (Thalussiosiru weissjlogii) removed the majority of the diatom organic material, leaving the siliceous frustule and the matrix-associated organic matter. This comparison is relevant because of documentation that copepods are one of the most abundant zooplankton on the Amazon shelf. The last column in Table 1 shows the mean zooplankton abundance for each of the AmasSeds cruises. A plot of zooplankton abundance vs phaeophytin standing crop has a negative slope and the linear regression has a coefficient of determination of only 0.36. A positive slope and a better correlation were expected between these parameters because of the phaeophytin production from zooplankton. A negative slope in the phaeophytim zooplankton regression suggests that micro-zooplankton (not sampled in the net tows) may be doing a significant amount of the grazing on the Amazon shelf. The ratio of phaeophytin standing crop to chlorophyll-a standing crop was equal to 0.5 or less for Cruises I, III and IV, inferring that the extent of zooplankton processing was less during these times than during Cruise II, when the ratio of phaeophytin standing crop to chlorophyll-a standing crop was greater than one. Throughout all four cruises, copepods were the dominant zooplankton group reported for most of the shelf, with the exception of some high abundances of crab larvae on the southern transect (ST-l) during Cruises I and III. Typical numbers of copepods in Amazon shelf surface waters were several hundred rnp3, with some values as high as 4600 individuals m -3 during Cruise IV. No estimates of zooplankton feeding rates were made during the AmasSeds Project. In the Columbia River estuary, Small et al. (1990) reported that macro-zooplankton grazing accounted for less than 3% of the phytoplankton removal on an annual basis.

Comparison of biological uptake rates with algal bloom dynamics: can the biology keep up with the physics? Phytoplankton growth on the Amazon shelf must be occurring primarily seaward of the high-turbidity plume, but shoreward of the nutrient-depleted offshore waters (i.e. in the optimal-growth zone). Nutrient uptake generally begins when the turbidity falls below 10 mg 1-l (DeMaster et al., 1983, 1986) and most of the bloom occurs shoreward of the 33 psu isopleth of salinity. Following the trajectory of the Amazon River plume, the lateral distance traversed by the phytoplankton during this bloom development is on the order of 200-300 km. Based on surface drogues (Limeburner et al., 1995) and ADCP data (Candela et al., 1992) the mean flow in the outer-shelf surface plume is on the order of 0.5-l .O m s-i. The phytoplankton, therefore, have approximately 2-7 days to develop their bloom while they are in transit across the optimal-growth zone (low turbidity, high nutrient waters). The biological production rates for silica and organic carbon provide an independent check on the physical measurements of plume flow and surface-water residence times. Cruise I data will be used as an example of this basic approach. For waters containing 10 mg suspended matter l-i, typical biogenic-silica contents are on the order of 1 pmol 1-i.

638

D. J. DeMaster

ef al.

DeMaster et al. (1991) and Nelson (1992) reported that these turbid waters have no net silica production. In the waters exhibiting a phytoplankton bloom during Cruise I, the standing crop of biogenic silica was as high as 1O-20 pmol I- ’, which requires at least four doublings during the 2-7-day transit across the optimal-growth zone (assuming that dissolution is negligible) or a doubling time on the order of 0.62 day-‘. Silica-rate measurements from the “green waters” of the optimal-growth zone (Nelson, 1992) yield a net silica production rate of 10-15 mmol m-s day-’ in surface waters that have a biogenicsilica abundance of about 8 mmol rn--’ (or 8 pmol 1-l). Therefore, the diatom doubling times from the silica-rate data are on the order of 1-2 day-‘, consistent with the values predicted from flow measurements and standing-crop data. The vertically integrated rate of total silica production at some of these green-water stations was as high as 110 mmol rn-’ day-‘, which is more than twice as high as values reported from highly productive, diatom-dominated coastal upwelling zones, such as those off the coasts of Peru and northwest Africa (Brzezinski and Nelson, 1989). 14C-production rates also can be used as a chronometer in this dynamic river/ocean mixing zone. The primary-production measurements on the Amazon shelf were based on incubation times of 24 h and, therefore, represent something between net and gross production estimates (Smith and DeMaster, 1996). In addition, some of the natural grazers may have been excluded during the sampling, which also may have affected the measured carbon uptake rate. During Cruise I, the suspended material at the bloom stations typically had an organic-carbon abundance ranging from 30-80 mmol mm3. Assuming minimal regeneration and grazing, this standing crop of organic carbon primarily must be produced during the 2-7-day transit across the optimal-growth zone. These data require a minimum primary-production rate between 4 and 40 mmol C rn-’ day-’ (or S&500 mg C m--3 day-‘). The measured primary-production values during Cruise I ranged from 1 to 2.50 mmol C rnp3 day-‘, with typical values from the “greenwater” bloom on the order of SO-160 mmol C rnp3 day-’ (1000-2000 mg C rnp3 day- ‘; Smith and DeMaster, 1995). Therefore, the 14C primary-production rates can easily explain the observed abundances of organic carbon in the surface waters and even require substantial bacterial degradation and/or grazing by zooplankton to keep the suspended organic-carbon concentrations from building up beyond the observed levels.

Comparison

with other estuarine

environments

Seaward of the high-turbidity zone, the extent of bloom development on the Amazon shelf appears to be controlled to a large degree by the nitrate flux from the river, as well as the residence time of surface waters in the optimal-growth zone (low turbidity/high nutrients). In the Mississippi River/ocean mixing zone, Dortch and Whitledge (1992) reported that riverine silicate and nitrate supply are important factors affecting the growth of algal blooms. The limiting nutrient in several of the Chinese rivers (i.e. the Yangtze and Huanghe) is phosphate, because of the large anthropogenic source of inorganic nitrogen (Edmond et al., 1985; Turner et al., 1990). Lohrenz et al. (1990) contend that bloom development within the Mississippi plume may be influenced by grazing and sinking, as well as by the short residence time of plume waters. Rapid mixing between riverine and oceanic waters also appears to be an important factor governing phytoplankton blooms near the mouth of the Zaire River (van Bennekom et al., 1978). These investigators also noted that nutrient uptake was light limited in this major African dispersal system, but the

Biogeochemical

processes

in Amazon

shelf waters

639

light limitation was the result of dissolved organic material rather than lithogenic matter, as is the case for most light-limited estuaries (Cloern, 1987, 1991). Pennock and Sharp (1986) examined phytoplankton production in the Delaware estuary over a period of 5 years and reported that the dominant regulator of primary production (annual average of 0.8 g C m-* day-‘) was light limitation because of high turbidity. Malone (1977) observed similar results in the lower Hudson estuary and noted that temperature also had a significant effect. Although there were extensive seasonal changes in the Amazon shelf during AmasSeds (e.g. riverine nutrient supply, winds and coastal current intensity), temperature remained nearly constant throughout the year, which is typical in tropical environments. During the four AmasSeds cruises, the standing crop of biogenic silica varied by only a factor of 2, the chlorophyll-a standing crop varied by a factor of 3.5 and the mean primary-production rate in the optimal-growth zone varied by less than a factor of 2 (2.2-3.7 g C me2 day-‘). Rates of organic-carbon remineralization in Amazon shelf sediments show little seasonality (Aller et al., 1996) consistent with a rather uniform supply of organic matter from the water column throughout the year. These relatively small changes in biogenic flux are in contrast with temperate river/ocean mixing zones like the Changjiang (Yangtze), where the mean primary-production rates vary seasonally by nearly two orders of magnitude (from 0.01 g C m-2 day-’ in January to 0.7 g C m-* day-’ in July; Ning et al., 1988). The depth-integrated rates of primary production on the Amazon shelf yield values on the order of 2.6 g Cm-* day-’ for the optimal-growth zone and mean values of -2 g C m-’ day-’ for the entire shelf region. To put these values in perspective, typical continentalshelf production rates are about 0.3 g C rnp2 day-’ for temperate environments and 0.6 g C In-* dayy’ for tropical areas (Kirk, 1983). Therefore, the nutrients supplied to the shelf by the Amazon River and the associated estuarine circulation (DeMaster and Pope, 1996) clearly enhance the rate of primary production relative to other shelf environments. Average rates of primary production in the plume of the Huanghe River vary from 0.2 to 0.5 g C m-’ day-’ (Turner et al., 1990), whereas the Changjiang (Yangtze) mixing zone exhibits values as high as 1.5 g C m-’ day-’ (Ning et al., 1988). In the Zaire, mixing zone rates of primary production tend to be low (
640

D. J. DeMaster et al.

CONCLUSIONS (1) In Amazon shelf surface waters, the distributions of surface pH, chlorophyll-a, oxygen saturation and the biogenic-silica content of suspended matter all showed a coherent pattern identifying areas of high productivity. 14C primary-productivity measurements were used to corroborate the locations of the high productivity areas, which generally occurred seaward of the 10 mg 1-l isopleth for suspended solids and shoreward of the 33-psu isopleth for salinity. (2) Based on the standing crops of biogenic silica and chlorophyll-a, as well as watercolumn depletions in silicate and inorganic nitrogen, bloom development on the shelf was greatest during the rising river discharge period (Cruise IT, March 1990). This extensive bloom occurred when the riverine nitrate flux was larger than during any other cruise and the residence time of surface waters on the shelf was the longest of any cruise, permitting phytoplankton maximum time in the optimal-growth zone. (3) The primary mode of cycling nutrients from the seabed to the water column appears to be seabed resuspension, which probably results from tidal processes and wave activity. The flux of silicate and inorganic nitrogen (nitrate, nitrite and ammonium) from the seabed was estimated by calculating the amount of excess nutrient on the inner-shelf (relative to the ideal mixing of riverine and oceanic end members) and then dividing that value by the residence time for inner-shelf waters. Based on expected residence times for the inner-shelf (-14 days) seabed resuspension provides on the order of 5-20% of the silicate and inorganic nitrogen reaching the outer-shelf plankton blooms. (4) Cruise IV (November 1991) was the only one of the four cruises in which the integrated silicate removal on the shelf (32 x 10s moles) was balanced by a comparable biogenic-silica standing crop in the water column (29 X 10s moles). During Cruise II (March 1990) the integrated silicate depletion (170 x 10” moles) far exceeded the biogenic-silica standing crop (66 X 10s moles), whereas during Cruise III (May 1990) a substantial biogenic-silica standing crop occurred (35 x 10” moles) despite the fact that there was a net addition of silicate to the shelf (5 x lOs), probably as a result of seabed resuspension. (5) The distributions of biogenic silica and silicate-depleted water are very poorly correlated on the Amazon shelf (coefficient of determination ~0.06). In general, the shelf waters exhibiting high biogenic-silica standing crops occur landward of the waters, exhibiting strong silicate depletions. This decoupling of the solid and aqueous phases of silica, which occurred during all four AmasSeds cruises, probably originates from siliceous particles sinking out of the photic zone (as a result of aggregation, grazing and/or silicate limitation) and being transported landward by the estuarine-like circulation. (6) Rates of primary production and silica production on the Amazon shelf are some of In the “optimal-growth zone” (
Biogeochemical

processes

in Amazon

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shelf waters

Acknowledgements-We would like to thank the crew of the R/V lselin and our Brazilian colleagues for their assistance in sample collection. We are grateful for the long hours put in by all of the principal investigators and by our graduate students and sea-going staff, who include: Robin Pope, Stephen Harden, Kevin Craig, Heinz Seltmann, Donnie Smoak, Holly Kelly, Linda Herlihy and Jim Rich. Nutrients were analyzed by D. Guffy of the Texas A&M Nutrient Research Group (D. Biggs, Director). Our manuscript benefited from the thorough reviews of R. Aller, C. Nittrouer, R. Whitlatch and E. Carpenter. This AmasSeds research was supported by grants from the National Science Foundation, Chemical Oceanography Program.

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