Date: Mon, 09 Mar 1998 11:17:07 -0500
Process 2 Report #3 64 deg. 42' S And the winner of the BLOOMIN' CONTEST (Latitude at which we would cross the peak of the bloom) was Rob Masserini with an entry of NEVER. As we steamed from 60 to 66 S (Mooring 3 to Mooring 5) pCO2 dipped a little in the same areas that it had during the four transects of Survey 2. Each transect had a smaller drawdown of CO2, and our minimum was 320 atm, suggesting a recovery from the bloom of earlier cruises by mixing of deeper water, etc. Between 66 and 69 S pCO2 remained high (340-360 atm) and Silica was between 55 and 60 mole/kg. Biomass was relatively high (150 g/l). Phaeocystis were abundant, unhealthy and clumping at some stations, suggesting the end of a bloom, but nitrate and phosphate remained high, though the N/P ratio reached a small maximum in this area (16.5 compared with 15 further north). Then pCO2 dropped rapidly to 264 atm as we steamed south to 70.3 S, and biomass increased to 230 g/l, but Silica remained high at 52-55 mole/kg, and nitrate dropped only from 25 to 20 mole/kg, so what has fueled the CO2 draw down? A Phaecocystis bloom? After the long station we steamed south as fast and far as we could before darkness halted our movement (2300) because of the omnipresence of large bergs on the radar and the desire not to hit small ones that weren't on the radar. There was always a bow watchman as our forward pair of eyes during movements in the foggy, southern reaches. pCO2 started to increase as we went from 70.3 to 71.2 deg. S and fluorescence and biomass decreased. Nutrients, including NH3 increased in this area, suggesting that we were moving south of what had been the bloom peak. The steep increase in Silica concentrations between 65 and 66 deg.S during this cruise matched very well the location of a similar increase just a month previously during Survey 2. Apparently the southward spreading "fire" burned out and we were left to examine the ashes. As Helen Quinby put it, we are seeing the trees in the fall without the leaves. What, then, was the nature and origin of the bloom to the south of 68 where CO2 had been drawn down so significantly? It was not a uniform southward progression as the ice melted south of this area. We speculated at the beginning of the cruise that if the ice melted over a large region rapidly we might see patchiness in the bloom that was related to eddy upwelling/downwelling and hydrodynamic effects rather than a southward progression. Blooms could spread zonally as well as meridionally, and that may be part of the answer here. The surface mixed layer decreased from 50-70 m at the northern end of the study area to only 20 m at the south, which should be favorable for a bloom. Much of the decrease may be due to lack of wind mixing. Within the Antarctic low pressure area the barometer readings were stable (970 mbar for the 3-4 days we were there), the winds were generally around 10 kts and the lack of swells spoiled us. Skies were constantly overcast and foggy. An interesting phenomenon for those of us looking for diel cycles in our data was demonstrated by the movement of an iceberg at our southernmost long station. At least three times during the two days the same iceberg insisted on moving through the small piece of ocean we were occupying. Gross tonnage and vulnerability determined the "winner." The ADCP suggested that only the surface water above the cold pool at 50 m was moving. Stephany Rubin's plots of pCO2, SST and salinity for the station revealed a periodicity of lateral gradients in the area and the inertial tide gently rotated the water with little advective current and brought the same water (and iceberg) past us several times - an aliasing to watch for in the final data analysis. At our southernmost station the nighttime radar showed a line of superbergs ten miles to the south that looked like the Aleutian Island chain. Visual observations in the morning light revealed that the "islands" were connected and formed a wall rather than a picket fence, though it did not seem to be attached to pack ice. We would have loved to go further south to reach pre-bloom waters, but it was hard to believe the "good" weather would hold out much longer and the thought of weaving our way north through the icebergs in rough weather, being restricted to non-foggy daylight hours for travel, compelled us to emulate Shackleton rather than Scott and turn back. At least we had seen the upturn in CO2 drawdown and perhaps some of the waters around 67 deg.S will prove to be prebloom waters anyway (or non-bloom this year). One of the exciting measurements that has helped guide us and characterize phytoplankton activity has been the Fast Repetition Rate (FRR) fluorometer and PDP flow cytometer measurements of Heidi Sosik and Rob Olson. The FRR (used in the underway flow-through mode and on shallow hydrocasts) provides values of total fluorescence and a measure of the physiological health of cells by assessing their ability to absorb and utilize light for photosynthesis - the variable chlorophyll fluorescence yield. This parameter indicates that the phytoplankton have been in moderate to poor physiological state over the whole cruise track, with surface values of 0.3 - 0.4 (where 0.67 indicates fully functional photosynthetic reaction centers), even though silica and other macronutrients have been abundant and there have been some periods of sun. Variable fluorescence yield decreased as we went south, to a low of 0.2 at station 13; but at station 14 (the southernmost station), it was again 0.4. In many depth profiles variable fluorescence increased near the bottom of the euphotic zone. In bottle incubation experiments, addition of 4 nM iron caused increases in variable fluorescence (as well as changes in absorption cross section and fluorescence decay constants) at three stations south of the Polar Front, suggesting that the phytoplankton were physiologically limited by iron availability. (Iron concentrations have been low (0.09-0.15 nM) for most of the transect according to measurements of Chris Measures.) Individual cell measurements suggested that pennate diatoms responded to iron enrichment with larger increases in variable fluorescence than other cells, and that both species and cell-to-cell variability in physiological state can be high. In general, our measurements suggest that large cells are in better condition than the "mean" phytoplankton cell in a given water sample. Primary productivity decreased from ~1400 mgC m-2 d-1 at Mooring 1 to <400 mgC m-2 d-1 as stations just south of the front (60 deg.S) - much lower rates than during Process 1. Chlorophyll has shown a similar trend with values of 0.2 mg m-3 south of the front around Mooring 4 compared with values of 1.5-2.5 mg m-3 during Process 1. Values increased further south, but are still low. Marra and company made a successful deployment and recovery of the PP array at the southernmost long station and plan to do so tomorrow in the area where the bloom was active during Process 1. Dickson and Orchardo report that net community oxygen production rates integrated to the 1% light level were the highest at the two stations occupied north of the Antarctic Polar Front at the beginning of the cruise (78 and 45 mmol O2 m-2 d-1). Between 60 and 66 deg.S oxygen production rates decreased markedly in the front and at two stations to the south of it to -22, -10 and -9 mmol O2 m-2 d-1, respectively. At the two southernmost stations located at 69 and 70 deg.S, positive net production rates of 2 and 3 mmol O2 m-2 d-1 were measured, although these values are not statistically different from zero. By way of comparison, the overall average net oxygen production during process cruise 1 was 89 mmol O2 m-2 d-1 compared to 12 O2 m-2 d-1 obtained so far during this cruise. Mesozooplankton abundance has remained high through the whole transect from 52 deg.S to 71 deg.S according to Juanita Urban-Rich and Jay Peterson. There appears to be a diurnal vertical migration with higher species diversity occurring at night. South of the polar front Calanoides acutus is the dominant copepod while north of the front Neocalanus tonsus is the prominent copepod. Mesozooplankton grazing rates increased south of 65 deg.S as chlorophyll and particle concentrations increased in the water column (average south of 65 deg.S; 20 pellets/copepod/day versus an average of 5 pellets/copepod/day north of 65). Krill were common in the night net tows at 70 and 71 deg.S. The SIO optics group (Rick Reynolds and John Wieland) continues measurements of inherent optical properties of the upper water column, and their relationship to planktonic biomass and particle size distribution. Progressing South from Mooring #1 to Mooring #4, particulate spectral absorption coefficients in the near-surface waters decreased more than 5-fold, increasing again towards the southernmost mooring. Throughout this transect, the relative contributions of phytoplankton and non-pigmented particles to total particulate absorption changed relatively little. Spectral scattering and backscattering coefficients have exhibited similar spatial trends as particulate absorption, but with a smaller range in the magnitude of variations (1.5-2.5 fold, depending upon wavelength). Chlorophyll a-specific optical coefficients have shown significant variability over the study area. The Cafe Thorium (John Andrews and Glenn Crossin) has analyzed the first three mooring stations for 234Th activity. Station 1 showed total Thorium close to equilibrium at all depths with activity on the 1-70 m particulate increasing towards the surface and >70 m particulate activity at a maximum at 50 meters and decreasing towards the surface and non-existent below 75 meters. Moving South to mooring 2 and 3 the total 234Th deficits are increasing towards the surface from 150 meters. By Mooring 3 the >70 m particulate activity has almost dropped to background and the 234Th total deficit is at a maximum at 25 meters, at 40% of equilibrium. Though we do not have carbon data from these cruises yet, the lack of >70 m particulate Th as we head South suggests the carbon export from the bloom has already occurred in these areas. The aggregate camera portion of the Particle and Optics Profiling System has been resurrected to partial life due to hours of concentrated work by Scott Hiller, SIO tech. Some profiles to 800 m as we head north will hopefully tell us about the size distribution of large particulate matter and provide clues about the pathway of carbon export from the surface. Some excellent profiles were made on the southward leg to about Mooring 4. We anticipated a rendezvous with the Palmer at Mooring #5, but it appears that will happen when we catch up with them further north. Every one continues to work hard and it is rewarding to see the data come in. We remind ourselves not to cement ourselves to shipboard hypotheses as many measurements await further laboratory analysis and are sure to add new dimensions to the picture. Wilf Gardner, Chief Scientist