JGOFS SMP REPORT
1997-2001
Synoptic Analysis of Factors Influencing Carbon Fluxes at the Cariaco Continental Margin Time Series
NASA
Principal
Investigator
Frank Muller-Karger
(carib@marine.usf.edu; phone: 727-553-3335; FAX: 727-553-1103)
College of Marine Science, University of South Florida
140 7th Ave. S., St. Petersburg, FL 33701
Partner Organizations:
Fundacion La Salle of Venezuela: In-kind Support; Facilities; Collaborative Research; Personnel Exchanges
Fundacion La Salle's Estacion de Investigaciones Marinas Isla Margarita (FLASA/EDIMAR) are active collaborators on this program, and they offered personnel, lab facilities, and ship time to conduct the work for this program.
Activities and findings:
The CARIACO (CArbon Retention In A Colored Ocean) study addresses the relation between near surface processes and the downward flux of particulate materials in the Cariaco Basin. The goal is to better understand the effects of past and present weather and climate change as recorded in the sediments of the Cariaco Basin. This site was selected to study carbon fluxes at a continental margin where down-slope lateral losses are minimal.
The field program has carried out monthly oceanographic observations at 10°30'N, 64°40'W since November 1995. Basic observations include hydrographic, dissolved inorganic and organic carbon concentration, particulate carbon and nitrogen concentration, phytoplankton composition, and primary productivity. Emphasis during the past 12 months (April 1998-March 1999) was on maintaining the monthly hydrographic time series, in improving nutrient concentration measurements, and in generating publications to report results obtained over the past 3 years.
The CARIACO activity was incorporated into the IGBP LOICZ Program in January 2000. The program continues substantial collaboration efforts with two other US institutions (University of South Carolina and State University of New York) and three Venezuelan institutions (Fundacion La Salle, Universidad Simon Bolivar, and Universidad de Oriente). Additional support was provided to MIT (E. Boyle) for whom we collected surface samples for trace metal research and to various other institutions that have requested CARIACO data. CARIACO also supported the NASA-funded Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS).
Through 2001, 3 graduate students worked on this program. Two undergraduate students were supported under this grant, with one graduating in December 1999. One undergraduate and one graduate student at institutions in Venezuela graduated with theses focusing on the CARIACO program.
Findings:
The study comprised four upwelling seasons and three non-upwelling seasons. Each upwelling season has been distinctly different while less difference was observed between the non-upwelling seasons.
1997-1998 REPORT:
1997-1998 was a very unusual year because of an absence of upwelling at in the Cariaco Basin. The two previous years of observation (1995-96, and 1996-97) featured strong upwelling from January through May. However, in the period from January 1998 through May 1998, upwelling occurred only in March. At that time, surface temperatures reached 22.7oC and chlorophyll concentrations at the surface reached values greater than 5 mg/m3. But the transition to- and from this state occurred very rapidly as surface temperatures in February and April were 25.8oC and 26.3oC, respectively. We believe this to be an El Niņo related phenomenon, as meteorological conditions in Venezuela showed strong anomalies marked by extreme drought during winter 1997-98.
1998-99 showed a return to the conditions observed in 1995-97. After the anomalous winter-spring, a brief period of upwelling was again found in July, 1998, suggesting that such brief summer phenomena are a regular occurrence in the southern Caribbean. This was followed by a typical fall non-upwelling period (high surface temperatures, a deep chlorophyll maximum at 50 meters). The 1998-99 upwelling season began as usual, with slight upwelling in November and December producing a chlorophyll maximum at 25 m. Upwelling developed fully by January, with the salinity maximum moving up to the surface and high chlorophyll observed throughout the upper 20 meters. These upwelling conditions have continued through March 1999.
During the 15 months preceding April 1998 (going back to January 1997) we observed five separate ventilation events of the intermediate waters (200-300 meters) below the sill depth at 150 meters. The first of these completely changed the upwelling season by causing deeper water with higher nutrients to rise toward the surface, resulting in primary production 50% higher during the upwelling season. During the last year we observed a ventilation in June 1998, but none in the 8 months since. In summary, over the life of the project we began with 14 months of no ventilations, then observed repeated ventilations over a period of 18 months, and now have had no ventilations for 8 months. This pattern remains unexplained but we are examining the far-field oceanography for possible connections under sponsorship of the JGOFS SMP program.
With the initiation of this grant we are collecting nutrient samples for measurement at USF in the lab of Dr. Kent Fanning. These measurements have revealed that nitrate, phosphate, and silicate are generally at undetectable levels in the surface waters during the non-upwelling season and only slightly higher during upwelling. This suggests complete utilization of nutrients during upwelling.
The higher quality measurements have also allowed us to examine the nutrients ratios in the Cariaco Basin. These ratios show evidence of vertical chemical fractionation, in which deep waters are enriched in carbon relative to nitrogen and phosphorus. For example, in deep water (400 m -1300 m), the C:P ratio is 127:1, while in intermediate waters (75 m -200 m), the ratio is 90:1. In addition, intermediate water shows enrichment in nitrogen relative to phosphorus, suggesting a history of nitrogen fixation. Combining this information on nutrient ratios with the observation that nutrients are almost completely utilized during upwelling suggests that carbon can be significantly drawn down by the biological pump during the upwelling season. This is exactly what we have previously observed with regard to fCO2, which is lower during upwelling and at times below the atmospheric levels.
Our synthesis of these data and results has begun and includes an assessment of the variability of fluxes in time. Results from the first three years of observations are being summarized in a series of manuscripts.
1999-2000 REPORT:
Emphasis during the April 1999-March 2000 period was on maintaining the monthly hydrographic time series, understanding the relationship to vertical carbon flux observations, and in generating publications to report prior results. Basic observations that have been continued for this year include hydrography, dissolved inorganic and organic carbon concentration, particulate carbon and nitrogen concentration, phytoplankton composition and pigment concentration, and primary productivity.
The year was marked by a return to a 'normal' seasonal pattern of upwelling and downwelling, i.e. one seen prior to the El Nino of 1997-1998. In 1999, the strongest upwelling was observed in February with surface temperatures of 22.5C. In March surface temperatures increased to 24.3 C and in April to 25.5 C, while the chlorophyll maximum moved from the surface to a broad peak between 20 and 60 m. The average primary production for January through May was 1,767 mg C m-2 day-1 which compares to an average of 1,784 mg C m-2 day-1 observed from January through April in 1995, our first field season. The non-upwelling season showed similar patterns to those observed previously every year, with a mild summertime upwelling in July and August. This was indicated by an increase in the concentrations within the subsurface chlorophyll maximum (a broad peak from 20-55 m). In October-November we observed the highest surface temperatures of >28 C while Surface salinities were the lowest observed during the year (~36.35 ppt). The salinity maximum was at 100 m at this time, with the chlorophyll maximum at 40-70 m. These are now recognized as typical pre-upwelling conditions. Upwelling did not begin until December 1999, when we had seen it usually start in November but also as early as October in previous years. Upwelling has continued into March 2000. No ventilation of the intermediate waters below the sill depth was observed during the year.
From the data collected this far, it appears that upwelling typically begins in November or December, peaks in February or March, and ends in May or June. We now consider 1996, 1999, and 2000 normal years, with average primary productivity around 1,800 mg C m-2 day-1. 1997 and 1998 were anomalous years. In 1997 ventilation of water at or beneath the sill depth raised the thermocline at the CARIACO station and resulted in the upwelling of cooler water with higher nutrients. This led to a higher average productivity of around 2,600 mg C m-2 day-1. In 1998, anomalous conditions seemed to be related to the previous year's strong El Nino event. That year, upwelling stopped in February, was strong again in March, but ended in April. This led to a low average productivity of 1,361 mg C m-2 day-1. We are currently looking at the larger scale applicability of these patterns using a variety of satellite data (AVHRR, SeaWifs, TOPEX), however, we anticipate that a more complete understanding will only come with a longer time series record.
2000-2001 REPORT:
During the last period of performance for this project we observed exceptionally strong upwelling in both the winter-spring main season (2001) and in the weaker summer season (2000). The resulting chlorophyll-a concentrations and primary production were among the highest recorded since CARIACO started in November 1995. The upwelling in January through April was also accompanied by substantial water movements below 200 meters that have not observed previously.
The winter-spring season in 2000 was similar to that observed in 1999 with surface temperatures reaching 23oC in February and May and an average primary production for January through April of 1,736 mg C m-2 day-1. This compares to average production of 1,897 mg C m-2 day-1 observed in 1999 and average production of 1,784 mg C m-2 day-1 observed in 1996 over the same months. Upwelling relaxed somewhat in March with surface temperatures increasing to 24.5oC and lower chlorophyll and primary production. Concentrations and productivity increased again in April and May. June showed the transition to non-upwelling conditions with a surface temperature of 25oC and a chlorophyll maximum at 35 m of only 0.35 mg m-3.
This situation reversed strongly in July with upwelling reducing the surface temperature to 23oC (as low as it had been at any point in the previous upwelling season) and the 21oC isotherm rising to a depth of 50 m from 80 m in the previous month. Chlorophyll concentrations rose to >1.0 mg m-3 in the upper 15 meters and primary production reached 1,800 mg C m-2 day-1. Both of these levels were higher than any previous observations for the summer upwelling period and are comparable to the winter-spring upwelling period.
The upwelling was short-lived and by August surface temperatures rose to 28oC and the chlorophyll at the surface had decreased an order of magnitude to 0.16 mg m-3. A broad chlorophyll maximum of 0.30-0.40 mg m-3 had developed between 35-75 m. These conditions of high surface temperatures, low surface chlorophyll, and a deep chlorophyll maximum continued through November. The average primary production for August through November was 805 mg C m-2 day-1 which compares to average values of 1,101, 726, 673, and 803 mg C m-2 day-1 measured in 1996, 1997, 1998, and 1999 over the July through November period.
Upwelling began again in December 2000, with surface temperatures decreasing to 25oC and productivity increasing to 1,680 mg C m-2 day-1. This trend continued and in early 2001 we observed the strongest upwelling season since 1997. Surface temperatures in February and March 2001 were less than 22oC. Chlorophyll concentrations were >3 mg m-3 throughout the upper 25 m for January through March---levels we had only seen before in May 1996, January 1997, and March 1998. With a mixed layer deeper than 25 m and these high chlorophyll concentrations, the euphotic zone was substantially smaller than the mixed layer depth. Nevertheless, average primary productivity for January through April reached 2,607 mg C m-2 day-1. This compares to an average productivity of 2,637 mg C m-2 day-1 observed in 1997, the previous maximum observed in the series of observations collected since November 1995.
Although the 2001 observations of surface temperature and productivity were similar to 1997, the water column structure showed a different cause for these phenomena. In 1997 we observed a ventilation event between 200 and 225 meters that raised the pycnocline and resulted in the upwelling of water with lower temperatures and higher nutrients at the CARIACO station. In 2001 we saw similar low surface temperatures and high production but there was no ventilation. Instead the water column to 250 m seemed to rise through upwelling. Previously we had seen very little upwelling effect below the Cariaco Basin sill depth, namely ~150 m.
The upward movement of large amounts of water in 2001 was clearly documented in several hydorgraphic variables. The oxygen sensor on the CTD showed that the oxycline, which extends from the sill depth downward to about 275 m depth, moved upward about 40 meters between January and March 2001. The oxygen concentrations decrease substantially at shallow depths in the Cariaco Basin through this massive upwelling. Dissolved oxygen typically had been ~1.83 ml L-1 at 160 m and ~0.64 ml L-1 at 200 m between 1996 and 2000. In March 2001, dissolved oxygen at 160 m decreased to 0.44 ml L-1 and at 200 m it decreased to 0.10 ml L-1. While the upwelling in 2001 may have been effected by a deeper Ekman layer, there may have been other processes acting to bring deeper water to the surface. In 1997, the large and repeated (but shallower) upwelling were only partly related to wind forcing. Using satellite altimeter data, we documented that in 1997 there were several eddies in the southern Caribbean Sea which led to ventilation in the Cariaco Basin. Similar analyses need to be conducted to understand the processes leading to strong upwelling in 2001.
If water from below the sill depth was upwelled in 2001, then what replaced it? In January through March we detected no intrusions of oxygenated water below the sill. However, in April the entire water column from 150 m to 250 m was replaced with oxygenated water (dissolved oxygen >0.6 ml L-1). Within this zone were two peaks with higher dissolved oxygen, indicating intrusion 'tongues'. Of particular interest will be the effect on the suboxic and anoxic microbial processes being studied by the SUNY group, and on the vertical flux of particulate organic carbon being studied with the USC group.
Other Specific Products:
Data or databasesThe CARIACO time series was started in November 1995 and is located at10.50 N, 64.66 W. It consists of a mooring with 4 sediment traps (200,400, 800, 1200 m; bi-weekly sample integrations), an upward-looking Acoustic Doppler Current Profiler (ADCP, 200 m; 1996-1998), and monthly cruises to examine the composition and light absorption properties of organic particulate and dissolved matter, the taxonomyof phytoplankton and general classification of bacteria, biological productivity (phytoplankton and bacteria), physical/chemical properties including nutrient, oxygen, and the carbonate system, and hyperspectral reflectance measurements. These hydrographic data are regularly submitted to NOAA-NODC and the bio-optical data are submitted to NASA-SeaBASS.
The data are openly shared with other NSF researchers working on this program, and with any interested investigator or person. The data are being prepared for distribution via open ftp access through our web page at: http://imars. usf.edu.
Internet Dissemination:
http://imars. usf.edu
This page incorporates 1) a history of activities carried out under the CARIACO program, including a pictorial album of activities 2) a discussion of results and summaries of publications, and 3) ftp access to the quality-controlled, core observations collected under the CARIACO program.
Contributions:
This program contributed unique observations and understanding on the magnitude and variability of carbon and nutrient fluxes in the ocean but at a continental margin site, as opposed to the current deep ocean sites. The multi-year series of observations allowed us to connect productivity in near-surface waters on a continental shelf to the downward flux of particulate material.
As part of our routine observations we measured the effects of an underwater 'landslide', caused by a strong earthquake on an adjacent coast. This allowed us to quantify the amount of material that can be buried in sediments deep in the ocean in addition to the flux of particles raining from the surface to the bottom.
We examined the N* (N-star or excess nitrogen relative to phosphorus in the Redfield sense) in Cariaco waters, and consider that recent publications which conclude that sediments in the Cariaco Basin reflect local nitrogen fixation may be in error. While these reports suggest that nitrogen fixation at this location is enhanced during interglacial periods because of denitrification in anoxic waters, we consider that the N* signal observed within the basin is imported from the North Atlantic central gyre. While denitrification takes place within hypoxic waters of the Cariaco Basin, it is deep and probably occurs primarily below the layer of inflowing waters that are subsequently upwelled at the CARIACO site.
Contributions to Other Disciplines:
The CARIACO site provides unique data for calibration of NASA ocean color satellites, specifically the SeaWiFS and soon the MODIS sensor to be flown on the Terra (EOS-AM1) satellite. Data from this program are also being used by Japan for calibration of their ocean color sensor, ADEOS-OCTS, flown from 1996-1997.
Preliminary findings suggest that the CARIACO data reflect major changes that occur in the North Atlantic, specifically as a result of El Nino and the North Atlantic Oscillation. These results may be relevant to physical oceanographers and climate modelers. We are actively looking at these results under the JGOFS SMP program.
Our observations on excess nitrogen relative to phosphorus in the dissolved inorganic nutrient pool (in the Redfield sense) indicate that recent reports identifying nitrogen fixation as an important local process which occurs in reponse to high denitrification rates within the Cariaco Basin are in error. Instead, we believe that this excess nitrogen is imported into the basin from the North Atlantic Central gyre, where nitrogen fixation indeed may play a major role.
Contributions to Education and Human Resources:
Our primary contribution to education and human resources was in the training of 1) a group of Latin American scientists in modern oceanographic techniques and the understanding of the value of continuous oceanographic measurements, 2) the training of primary and middle school teachers participating in Project Oceanography and the Summer Camp for Teachers at USF. The latter program exposed local Florida teachers to techniques used for measuring fluxes in the ocean, and taught them of the importance of these observations for local and global scale science, and 3) training of undergraduate and graduate students and post-doctoral research associates.
Contributions to Resources for Science and Technology:
This project provided a unique data set for assessing the impact of changes that take place in the ocean. The site chosen for routine oceanographic sampling is a sensitive location in which changes that occur in the tropical North Atlantic seem to be magnified. The observations collected represent the longest series of comprehensive oceanographic measurements at a continental margin tied with bio-optical (satellite calibration) observations and concurrent vertical particulate and carbon flux measurements using sediment traps.
The CARIACO study has been selected to support the IGBP LOICZ (Land-Ocean Interactions in the Coastal Zone) Program, and as such contributes a unique time series to this international program.