Primary productivity and phytoplankton pigment measurements in the North Sea

During the culmination of the phytoplankton spring bloom in the Fladen Ground area in April-Mai 1976, gross primary production was between 1500 and 2000 mg particulate C m**-2 day**-1, at a crop density (mainly diatoms of the genus Chaetoceros) of about 1500-3500 mg C m**-2. Estimates of the C:chlorophyll a ratio in living cells were much lower than those reported in the literature, possibly because part of what is measured as "chlorophyll a" by the common fluorometric method is associated with particles that are not reported as cells. Most of the dark 14C fixation during the bloom's climax was due to abiotic processes. Excretion of 14C-labeled carbohydrates did not account for a significant fraction of the total photosynthetic rate. The low crop after the bloom period, in June, corresponded with nutrient depletion of the euphotic zone. The low photosynthetic efficiency in June may have been a gross underestimate.
The presence of relatively high concentrations of chlorophyll derivatives signifies that the algal crop was consumed by heterotrophs, but at a lower rate in April/May than during the June cruise when particularly high molar ratios of phaeophorbide a and phaeophytin a relative to chlorophyll a were found. The high respiratory rate relative to autotrophic production in June manifested itself also in high dark 14C fixation values. The high concentration of phaeophorbide a in the upper 40 m and its scarcity below this depth during the spring bloom climax in April/May implies that copepod grazing at that time took place principally in the euphotic zone. The remarkably high concentration of chlorophyllide a in the surface layer during the bloom period indicates that the part of the crop that was destroyed by the grazers while eating was occasionally as high as the part that was actually ingested.

1976年4—5月，在弗拉登海域（Fladen Ground）春季浮游植物水华（phytoplankton spring bloom）盛期，总初级生产量（gross primary production）介于1500~2000 mg颗粒态碳（particulate C）·m⁻²·d⁻¹之间，此时浮游植物现存量密度（以角毛藻属（Chaetoceros）硅藻为主）约为1500~3500 mg C·m⁻²。活体细胞内碳与叶绿素a（chlorophyll a）的比值估算值远低于文献报道结果，这可能是因为常规荧光光度法（fluorometric method）所检测的“叶绿素a”中，有一部分附着于非细胞态颗粒之上。水华盛期绝大多数暗环境下的¹⁴C固定过程均由非生物途径介导。浮游植物对¹⁴C标记碳水化合物的排泄量，并未占总光合速率的显著比例。水华结束后6月的低现存量，与真光层（euphotic zone）的营养盐耗竭相对应。6月的光合效率估算值可能存在严重低估。
叶绿素衍生物（chlorophyll derivatives）浓度相对偏高，表明藻类生物量已被异养生物（heterotrophs）摄食，但4—5月的摄食速率低于6月航次——该航次中脱镁叶绿酸a（phaeophorbide a）与脱镁叶绿素a（phaeophytin a）相对于叶绿素a的摩尔比值显著偏高。6月呼吸速率相对于自养生产的占比偏高，这一特征同样体现在较高的暗环境¹⁴C固定值上。4—5月春季水华盛期，表层40米以浅水域脱镁叶绿酸a浓度较高，而该深度以下几乎无该物质分布，这意味着彼时桡足类（copepod）的牧食活动主要发生在真光层内。水华期表层水体中叶绿素酸a（chlorophyllide a）浓度异常偏高，表明摄食者在取食过程中破坏的藻体部分，偶尔可与实际被摄食的藻体部分相当。