Phytoplankton chlorophyllain Lake Poyang and its tributaries during dry, mid-dry and wet seasons: a 4-year study

Abstract

Key-words:Lake Poyang, tributaries, chlorophyll a, light, water retention timeVariations in phytoplankton chlorophyll a were studied in Lake Poyang, and these variations were compared with those observed in its tributaries. Samples were collected from January 2009 to January 2013 during the dry, mid-dry and wet seasons. The study showed that chlorophyll a concentrations were significantly higher in the wet season than in the dry and mid-dry seasons (p < 0.0001) in the lake; a comparative analysis of Lake Poyang and its tributaries showed significant differences in chlorophyll a between the two ecosystems during the wet season but not during the dry and mid-dry seasons. During the wet season, the underwater light conditions were significantly elevated and water retention time increased substantially (25.5 d). In combination, these factors contributed to a significantly higher chlorophyll a concentration during the wet season than that during the dry and mid-dry seasons in the lake; the effect of nutrients on the phytoplankton was not obvious. The large variations in water retention time during the three seasons indicated that Lake Poyang has the combined characteristics of a river and a lake. The short retention times during the dry and mid-dry seasons produced no difference in chlorophyll a between the lake and its tributaries, whereas the long retention time during the wet season facilitated phytoplankton growth in the lake. ces facteurs a contribué à une augmentation significative de la concentration en chlorophylle a pendant la saison des pluies ; l'effet des nutriments sur le phytoplancton n'était pas évident. Les grandes variations dans le temps de rétention de l'eau au cours des trois saisons impliquent que le lac Poyang combine les caractéristiques d'une rivière et d'un lac. Les temps de rétention courts pendant les saisons sèche et mi-sèche n'entraînent aucune différence en chlorophylle a entre le lac et ses affluents, tandis que le temps de rétention long pendant la saison des pluies facilite la croissance du phytoplancton dans le lac.

Investigation of Residence and Travel Times in a Large Floodplain Lake with Complex Lake-River Interactions: Poyang Lake (China)

“…The waters adjacent to the lake's catchment river mouths are renewed considerably more quickly, with residence times of shorter than 2 days (Figure 7a,d), due to the stronger flushing of the catchment river inflows (Figure 8). From the upstream gauging station of Kangshan to the lake outlet of Hukou, the tracer concentrations along this main flow channel and its floodplain drop below the e-folding time after approximately 20-30 days following its introduction (Figure 7b), which is consistent with the average value of 26 days (in July and August) found by previous study [26]. However, areas characterized by slightly higher residence times are located in the downstream flow channels of the lake (Figure 7a), which is mainly attributed to the strong effects of backflow from the Yangtze River on northern Poyang lake (Figure 8) [17,27].…”

“…In general, this study delivers the estimates of the same order for this high water level period. Although no observation was made to confirm the spatial pattern of the residence times, the locations and magnitudes of the concentrations of water environmental factors can be explained, in part, by the "residence time distribution" phenomenon [5,26,32,46]. Figure 9 shows the spatial distribution of algae biomass averaged over 83 field samplings during the high water level period (in July 2012) of Poyang Lake.…”

“…These urgent problems concerning Poyang Lake have recently gained considerable attention [23,24] because they have resulted in a serious threat to inhabitants that rely on this freshwater resource [25]. The lake water quality is known to exhibit significant temporal and spatial variations that are closely related to lake-dependent flushing [21,22,26]. Therefore, it is important to determine the residence and travel times within Poyang Lake, particularly with respect to its seasonal water level dynamics.…”

“…Previous efforts have focused on estimating the average residence times of Poyang Lake by using a single value based on the ratio of the lake volume to the outflow rate [26][27][28][29]. In its simplest form, this method may not be appropriate for systems that are similar to Poyang Lake, which have highly variable volumes (varying from 4.5 × 10 ) and outflows (the lake discharges to the Yangtze River but with frequent backflow) due to complex water level dynamics [17].…”

“…However, field experiments that infer the residence time are expensive; thus, the overall spatial and temporal data cannot be surveyed [32]. Due to large variations in the published values of the residence times (varied from 10 to 59 days) [26][27][28][29], approaches for more accurate and realistic expressions that identify and quantify the spatially heterogeneous phenomena of residence time are required for this highly dynamic lake. Therefore, this particular case can be considered to be a first study.…”