Research On The Packing Effect Of Marine Phytoplankton And Its Bio-optical Impact

Phytoplankton are the main contributor to the ocean's primary productivity,relying on photosynthesis to exchange materials with the outside environment.The ability to absorb light energy,as inherent optical properties,is one of the prerequisites for photosynthesis.Thus,exploring its variation is crucial to understanding their growth process.The phytoplankton pigment package effect(Q_a^*)is an important optical parameter that affects the phytoplankton absorption coefficient by controlling the magnitude of the absorption spectrum and plays a vital role in the bio-optical progress of phytoplankton cells.Thus,it is an essential part of ocean color research.However,the existing quantification methods of Q_a^*are still incomplete,and most studies are based on datasets collected and measured in local sea areas and carry out research on specific bands and thus miss the understanding of that in the global oceans.Compared with the in situ observations,satellite remote sensing can provide long-term series datasets with a high temporal resolution,wide sight,and easy-access advantages,which forms the data basis for global ocean phytoplankton research.However,it still faces many difficulties and challenges:(1)how to optimize and improve the quantification model of Q_a^*in typical sea areas based on the dataset of phytoplankton pigment and absorption coefficient;(2)how to quantify the contribution ratio between Q_a^*and related variables for global oceans.Therefore,based on the global field-measured datasets,the pigment compensation model was first established and analyzed the global distribution modes of Q_a^*,and other relevant parameter models were further deduced.Combined with the MODIS-Aqua long-term series satellite dataset(2002–2020),we analyze the Q_a^*in the global ocean and its influencing factors,as well as the internal relationship between packing effect and phytoplankton absorption coefficient,and quantified the contribution ratio of one to each other.The main research conclusions include:(1)The quantification model of Q_a^*is improved and optimized,and product the satellite distribution successfully.Based on the in situ datasets of the China coastal waters(representing Case II water)and the Atlantic Ocean(representing Case I water),this study establishes a pigment compensation model,which successfully reduced the probability of abnormal Q_a^*(Q_a^*>1).The Q_a^*quantification model in Case II water shows good performance within1<TChla<4 mg m^-3.For example,in 520 nm,the proportion of normal Q_a^*(0<Q_a^*<1)increased from 70%to 100%;Meanwhile,in Case I waters,when TChla>0.1 mg m^-3,Q_a^*rose significantly from 40%to 90%in the 455 nm band.Finally,the successful satellite application filled the gaps in the application of the package effect and showed good consistency between Q_a^*and TChla concentration,i.e.,the increase of TChla corresponds to a stronger package effect(Q_a^*?0).(2)Quantitative analysis reveals the internal relationship between Q_a^*and TChla or equivalent size index in global oceans.Based on the global long-term series satellite dataset,correlation and causality analysis were used to quantitatively express the effects of TChla and equivalent particle size(SI_t)on Q_a^*and to clarify the role of Q_a^*in different phytoplankton bio-optical processes.The results show that:(1)the distribution of absolute causality:there are sparse and weak causality signals in the middle latitude of the Northern Hemisphere(30°N–55°N,-180°E–180°E)and in the western Pacific Ocean(15°S–30°N,100°E–180°E),indicating that the authenticity of the correlation in these areas is weak.However,in other low-latitude regions,Q_a^*has a strong causal relationship with TChla or SI,but is asymmetric.These regions correspond to the negative correlation between Q_a^*and TChla or SI.Differences in the distribution of normalized causality:Q_a^*contributed about 10%–50%to the variation of TChla and SI_t,which was shown to stabilize the variation of TChla and SI_t.TChla and SI_t contribute about 40%–60%of the changes in Q_a^*and bring greater unpredictability.(2)The balanced progress between Q_a^*,TChla,and SI_t:combined with the global distribution of solar radiation energy,the annual fluctuation of light intensity in low latitudes is slight,and the balanced progress between Q_a^*,TChla,and SI_t is mainly driven by phytoplankton.In the mid-latitude region,the irradiance-driving mode is observed;the sea surface light intensity has a wider range of variation.When the irradiance increases,phytoplankton cells will increase the packing effect(Q_a^*?0)to capture fewer photons and protect photosynthetic organs for continuous photosynthesis.(3)The effect of package effect on phytoplankton absorption coefficient(a_ph(?))was preliminarily quantified.The results show that:(1)the information flow(IF)from package effect to a_ph(?)is weak in the middle and high latitudes of the Northern Hemisphere(positive distribution).However,it is distributed in the low latitudes(negative distribution),while the IF from a_ph(443)to Q_a^*(443)(T_aph?Qa*)shows an opposite trend.The normalized IF revealed that Q_a^*contributed 20%–50%of a_ph(443)changes in the lower latitudes,stabilizing a_ph(443)changes.The IF in the northern hemisphere was slightly weaker than the southern hemisphere,while the Southern Ocean and the northern North Atlantic contributed 30%–40%of a_ph(443)changes,introducing more uncertainties to a_ph(443).(2)Time-delay analysis of absolute IF showed that when a_ph(443)lagged Q_a^*by one month,T_Qa*?aph was still weak,but the intensity was stronger than that in the same time period,and the distribution modes were similar.When Q_a^*lagged a_ph(443)by one month,T_aph?Qa*showed negative information flow in global sea areas;the phytoplankton absorption with advanced changes tended to stabilize Q_a^*and pigment composition changes,reducing their uncertainty.