Bridging the euphotic zone depth between optics and biology

Key Points: a) I_c corresponds to ~ 0.94% of surface USR and ~ 0.52% of surface PAR, respectively. b) The compensation depths are always deeper than Z_1%^PAR and shallower than Z_1%⁴⁹⁰, but quite comparable with Z_1%^USR in SCS and North Atlantic. c) As a rule of thumb, the 1%-USR depth is more appropriate in representing the compensation depth and determining the base of the euphotic zone than the 1%-PAR convention, at least for the waters under study.

Abstract: There have been debates in the past few decades that the conventional optically determined euphotic zone depth (Z_eu, a depth of 1% surface PAR (Z_1%^PAR)) appeared not matching the compensation depth (Z_c, ~ 40 – 140 m), i.e., the euphotic zone of biological significance. In this study, based on measured profiles of solar radiation and primary production in the South China Sea (SCS) and the North Atlantic, the compensation depth was used to evaluate the profiles of PAR and the usable solar radiation (USR) (Lee et al. 2014). It was found that at Z_c the light intensity is generally 0.52% (± 0.13%) of surface PAR, while it is generally 0.94% (± 0.17%) of surface USR, and much more stable than the ratio of PAR. These results indicate that Z_1%^PAR is too shallow (~19.5% shallower) to represent the base of the euphotic zone where phytoplankton photosynthesis exist, at least for the waters under study. More interestingly the depth of 1% surface USR (Z_1%^USR) appeared closely matching Z_c (~3% difference on average). These results suggest that Z_1%^USR is a promising optical parameter to bridge the euphotic zone depth between optics and biology, at least for tropical-subtropical waters.

Station	Ic/PAR(0)	Ic/USR(0)	Ic	Zc	Zmax	Z1%PAR	Z1%USR	Z1%490
	(%)	(%)	μmol m-2 s-1	(m)	(m)	(m)	(m)	(m)
LE04	0.6	1.2	10.1	84	60	75	84	108
E505	0.5	1.1	8.0	93	50	79	95	125
S504	0.7	1.1	4.8	92	60	84	94	123
E607	0.4	1.0	5.9	65	65	54	65	76
S501a	0.4	0.9	6.1	60	-	48	60	69
S502	0.4	0.9	4.4	60	48	49	58	65
E404	0.5	1.2	3.5	61	58	46	58	67
E404	0.5	1.0	8.0	92	62	75	91	120
LE06	0.5	1.0	5.5	92	79	81	90	116
C1	0.4	1.0	11.9	82	63	69	81	100
16	0.8	1.1	4.5	37	-	33	37	42
17	0.6	0.9	9.8	66	-	59	64	74
20	0.7	1.1	2.7	95	-	88	96	109
22	0.4	0.7	4.3	137	120	106	126	137
25	0.6	1.0	3.2	97	96	91	96	117
26	0.8	0.9	5.0	90	58	75	83	87
27	0.4	0.5	2.0	76	-	55	66	74
28	0.5	1.1	8.8	59	-	52	60	69
29	0.5	0.8	6.4	70	52	62	67	75
30	0.5	0.7	2.9	57	40	46	54	54
31	0.4	0.7	5.1	37	26	29	34	37
Mean	0.5	0.9	5.8	76	61	65	74	88
Stdev	0.1	0.2	2.7	22	9	20	22	28