Hypoxia In The Changjiang Estuary And Its Adjacent Area

We established the HPLC method of phytoplankton pigments analysis in our lab and measured the samples from Xuliujing, the Changjiang estuary, East China Sea (ECS) and a sediment core. The distribution and dynamics of POC in the Changjiang estuary and ECS were discussed based on the data from 1999 to 2003, in order to shed light on the background of the study area. Further, based on the field observation data and a culture experiment, hypoxia in the Changjiang estuary and its adjacent area were studied.Averaged POC value in the Changjiang estuary, continental shelf and outer area where Kuroshio prevailed are 26.5, 7.7 and 3.3μM, respectively. Elevated TSM (117mg/L) were observed at the bottom, probably due to resuspension. At E4 station, continuous survey for 24h suggested that POC value varied periodically, following the pattern of a sinusoidal curve. Observed curve fit well with the assumed theoretical sinusoidal curve, with periodic of 13h, consistent with the tide. Residence time of POC at E4 station is on the order of weeks, increasing eastwards. POC flux through the nepheloid layer to the Okinawa Trough was estimated to be 0.22 × 10¹²g/yr. ～2% of the Changjiang POC flux.As indicated by the survey at Xuliujing, chlorophyll a concentration in 2006 increased 0.2μg/L than that in 2005, averaged 0.9μg/L Other pigments ranged from 0.02μg/L to 0.32μg/L, with fucoxanthin 0.3μg/L, peridinin 0.1μg/L. Seasonal variation of chlorophyll a is not clear, while as for the other pigments, difference among seasons is within 100%.Chlorophyll a, fucoxanthin, peridinin and chlorophyll b averaged 1.3μg/L, 900ng/L, 290ng/L and 64ng/L from June to October, 2006. In respect of the diagnostic pigment, peridinin was higher in June and elevated fucoxanthin was found in August and October. Elevated chlorophyll a is found at 123°E, decreased both eastwards and westwards. Perid/Fuco ratio elevated at 122.5 °E, decreased with distance from the Changjiang estuary. Pigment mainly distributed at surface and subsurface layer, and depleted beneath 40m. In the cruise of August, 2005, which covered an area closer to the Changjiang, chlorophyll a, fucoxanthin and peridinin mean value were 1.3μg/L, 1.3μg/L and 0.2μg/L, respectively. As to the Feberary, 2007, which covered the whole ECS shelf, pigment values were generally lower than that in summer time, with mean value of chlorophyll a, fucoxanthin, peridinin and chlorophyll b being 0.6μg/L, 240ng/L, 40ng/L and 76ng/L DV chlorophyll a was observed at the southeast part, indicating the existence of Prochlorococcus.Based on the field observation data, hypoxia occurred in summer time, between 122～123.5°E and 30～33°N. Average bottom DO value decreased from 5.9mg/L (1959) to 2.7mg/L (2006), with area increased from ～1800km² (1959) to 15400km² (2006). In August, 2006, vast area of DO less than 3 mg/L also was observed. The condition in 2006 was worse than that as reported in 1999. A comparison with former study reveals that the hypoxia area is moving northwards.Elevated DO value was accompanied with lower POC/chlorophyll a ratio. DO decreased with increasing POC/chlorophyll a value, indicating that low DO occurred when the organic matter contained less fresh content.Analysis with other parameters reveals that AOU is correlated withorganic matter decay: the AOU is reversely correlated with POC/NO3.Meanwhile, hypoxia showed good relationship with Δρ/ΔZ. Multiple analysissuggested that AOU is closely correlated with stratification and organic matter decay. No clearly relationship was found between AOU and temperature in summer. Thus hypoxia in the Changjiang estuary is mainly the result of stratification and organic matter decay. The AOU could then be quantitatively expressed as follows:AOU=2.5+0.0015 × T²+0.2570 × Δρ-0.3730 × In(POC/NO₃)An organic matter decay experiment was carried out on board in October, 2006, with condition controlled as low DO (DO%<50%) and high DO (DO%>95%). Pigments decreased apparently before and after the experiment. In the low DO condition, chlorophyll a and fucoxanthin decreased from 0.8μg/L and 1.0μg/L to 0.3μg/L and 0.5μg/L In the high DO condition, chlorophyll a and fucoxanthin decreased from 1 μg/L and 1.μg/L to 0.3μg/L and 0.3μg/L. POC value also decreased from 20.7μM(low DO), 28.6μM(high DO) to 18.0μM(low DO), 16.3μM(high DO), respectively. No clearly coupling between nutrients and POC were found probably due to the high nutrients background concentration. However, ammonia did increase through the experiment, probably due to the activity of bacteria, since bacteria increased from 910×10³/mL to 1300×10³/mL FCM-detectable phytoplankton decreased clearly through out the experiment, from 270×10²/mL(low DO), 200×10²/mL(high DO) to finally 20×10²/mL(low DO), 40×10²/ml_{high DO}. Under such a specific experiment condition (i.e. high nutrient, POC value ～20μM, low pigments initial concentration), Chlorophyll a and fucoxanthin showed clearly exponentional decay, with decay constant k (d^-1) of 0.22(low DO), 0.18(high DO) and 0.21 (low DO), 0.18(high DO), respectively. Chlorophyll a concentration half life is longer under low DO condition, indicating that pigments decay slower under low DO condition. Estimate based on the POC decay suggest that the oxygen demand rate is 1.7-1.9mmol m^-3 d^-1 in the Changjiang estuary in summer.Further analysis of sediment pigments reveals that chlorophyll a concentration of the surface sediment averaged 65mg/g OC in the Changjiang estuary and adjacent area, followed by fucoxanthin (38mg/g OC) and peridinin (17mg/g OC). Content of chlorophyll a, fucoxanthin and peridinin is depleted close to the Changjiang river mouth, surrounded by elevated area north and south. With regards to chlorophyll b, mean value is 14mg/g OC, with maximum 44mg/g OC. Pigment ratio of peridinin/fucoxanthin (0.46) and fucoxanthin/chlorophyll a (0.56) in the surface sediment is close to that (0.56 and 0.56, respectively) in the water column. Alloxanthin/chlorophyll a ratio in the sediment (0.23) differed much from that in the water column (0.01), probably due to the different stabilities. Distribution of chlorophyll a in the sediment is similar to the chlorophyll a and hypoxia in the water column, suggesting that elevated primary production and low DO concentration is propitious to pigments conservation. In respect to the E4 core, pigment showed elevated value at the surface and decreased dramatically with depth. Chlorophyll a ranged from 0.4～147mg/g OC. Averaged value of fucoxanthin, zeaxanthin, alloxanthin were 6.3, 11 and 7.3 mg/g OC, respectively. Chlorophyll a distribution in sediment core showed good agreement with that of Bio-silicate, in situ chlorophyll a content and the Changjiang water discharge. Dramatical increase of pigments at the upper part of the core, coincide with the history of eutrophication of the Changjiang and increasing blooms in the area. Modeling analysis suggests that chlorophyll a decayed in the core following an exponential pattern (r²=0.94), with decay constant k=1.39yr^-1. Considering the exponential decay process, chlorophyll a still showed increase trend after 1980s, indicating the water column was more suitable for pigments conservation after 1980s.