20th Apr, 2017

Spring bloom well underway at L4

There seems to be a distinct difference now between the bloom reported on nearly 2 weeks ago (see post from 7 April) and now, in that the level of nitrate is dropping away rapidly.  Although the instrument (Satlantic SUNA) has not had a post field calibration applied, the values have dropped some 3 – 4 uM in the past two days, indicative of phytoplankton consumption of the nutrients.  The levels of chlorophyll at 06:00 this morning were >4 mg/m3; the issues of high-light quenching of the fluorescence close to the surface is a known problem, especially on clear sunny days between mid-morning and early evening.  For the latest from the L4 buoy data check out the figures below.

oxy_fluor_turb

 

nitrate_cdom

 

The CTD data is also clearly showing surface stratification with Tuesday’s profile (18 April) showing a top mixed layer extending down to 20m which is 0.5 degC warmer than the water beneath.

Responses

From Glen Tarran and Claire Widdicombe:

Interestingly, at E1 yesterday I got what I guess to be single celled Phaeocystis at maximum abundance down at 60 m, which, at L4 generally indicates the end of the Phaeocystis season. At L4 on Tueseday, Phaeocystis single cells were still not very abundant/clearly visible on the flow cytometry plots and more abundant in the upper 10m, suggesting that the Phaeocystis season here is only just underway.
Going back to E1, Claire was surprised to find that the net sample contained a lot of dinoflagellates and fewer diatoms than expected. Perhaps characteristic of post-bloom conditions at E1?
At L4 Claire found lots of diatoms, dominated by Guinardia delicatula, more typical of a spring bloom, although ‘typical’ is a bit of a loose term, particularly for L4.

From Claire Widdicombe:

I noticed the bloom starting two weeks ago but unfortunately we didn’t have a net sample last week. Tuesday’s L4 net sample was teeming with the diatom Guinardia delicatula and I’ll analyse the 10m sample on Monday to calculate total abundance. Surprisingly yesterday’s E1 net sample was not dominated by Guinardia but by large thecate dinoflagellates of the genus Protoperidinium (and others). This puzzled me, but Paul has analysed the E1 10m sample from 2 weeks ago and it was full of Guinardia delicatula (24,000 cells per litre) so I guess the bloom has already passed through E1 and has now arrived at L4?

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