Spatial patterns and environmental associations of deep scattering layers in the northwestern subtropical Pacific Ocean
Abstract: The mesopelagic communities are important for food web and carbon pump in ocean, but the large-scale studies of them are still limited until now because of the difficulties on sampling and analyzing of mesopelagic organisms. Mesopelagic organisms, especially micronekton, can form acoustic deep scattering layers (DSLs) and DSLs are widely observed. To explore the spatial patterns of DSLs and their possible influencing factors, the DSLs during daytime (10:00–14:00) were investigated in the subtropical northwestern Pacific Ocean (13°–23.5°N, 153°–163°E) using a shipboard acoustic Doppler current profiler at 38 kHz. The study area was divided into three parts using k-means cluster analysis: the northern part (NP, 22°–24°N), the central part (CP, 17°–22°N), and the southern part (SP, 12°–17°N). The characteristics of DSLs varied widely with latitudinal gradient. Deepest core DSLs (523.5 m±17.4 m), largest nautical area scattering coefficient (NASC) (130.8 m2/nmi2±41.0 m2/n mile2), and most concentrated DSLs (MGL, 6.7%±0.7%) were observed in NP. The proportion of migration was also stronger in NP (39.7%) than those in other parts (18.6% in CP and 21.5% in SP) for mesopelagic organisms. The latitudinal variation of DSLs was probably caused by changes in oxygen concentration and light intensity of mesopelagic zones. A positive relationship between NASC and primary productivity was identified. A four-months lag was seemed to exist. This study provides the first basin-scale baselines information of mesopelagic communities in the northwest Pacific with acoustic approach. Further researches are suggested to gain understandings of seasonal and annual variations of DSLs in the region.
Figure 2. Echogram (38 kHz) after noise-removal showing the DVM of mesopelagic organisms (at local time 0:00–24:00 September 10, 2018). The red lines indicated the period of midday (at local time 10:00–14:00), when the dwelling depth of mesopelagic organisms was relative stable and it was easy to eliminate the disturbance of DVM. And the black lines indicated the period of midnight (at local time 22:00–next day 2:00), The black arrow indicated the core part of deep scattering layers (DSLs), and the white arrow indicated the diffuse part of DSLs.
Figure 4. Maps of environmental conditions in the study area (the average data during September was used for mapping). a, b. Sea surface temperature (SST) and mesopelagic average temperature (MAT); c, d. sea surface salinity (SSS) and mesopelagic average salinity (MAS); e. mixing layer depth (MLD); f. mesopelagic average dissolved oxygen concentration (MAO); g. net primary productivity (NPP); h. 490 nm light attenuation coefficient (LAC). The black curves in some images are isolines.
Figure 6. Box plots about the features of environment in three parts (NP, northern part; CP, central part; SP, southern part). a. Mesopelagic average temperature (MAT); b. mesopelagic average salinity (MAS); c. mesopelagic average dissolved oxygen concentration (MAO); d. 490 nm light attenuation coefficient (LAC); e. primary productivity (PP).
Figure 7. Box plots about the features of deep scattering layers (DSLs), showing the differences among three parts (NP, northern part; CP, central part; SP, southern part). a. Mesopelagic nautical area scattering coefficient (MNASC); b. center mass (CM); and c. mesopelagic gathering level (MGL) of mesopelagic zone. d and e showed the upper (UBD) and lower boundary depth (LBD) of DSLs, respectively.
Figure 8. Vertical distribution of acoustic backscatter grouped into three parts, according to the result of k-means cluster analysis. a, northern part; b, central part; c, southern part. Red lines show average midday-time profiles (10:00–14:00), and light red shadows are range of standard deviations. Black lines show average midnight-time profiles (22:00–next day 2:00), and gray shadows are range of standard deviations. x-axis is acoustic backscatter measured as nautical area scattering coefficient (NASC) at the corresponding depth.
A1. Midday vertical distributions of backscattering strength and its changing gradient in the whole study area. a, b. The vertical distributions of increasing gradient and decreasing gradient, respectively; c. the vertical distribution of mean volume backscattering strength (MVBS). Black curves are identified upper and lower boundaries of DSLs (UBD and LBD). x-axis is the column data number.
Table 1. List of abbreviations in the present study
Acronyms Full names Unit NASC (sA) nautical area scattering coefficient m2/n mile2 ASC (sa) area scattering coefficient m2/m2 MGL mesopelagic gathering level % VBC (sv) volume backscattering coefficient M−1 MVBS (Sv) mean volume backscattering strength dB re 4 II m−1 CM center mass m UBD upper boundary depth m LBD lower boundary depth m MA migration amplitude n mile2/m2 MP migration proportion % WMD weight migration depth m LAC light attenuation coefficient m−1 NPP net primary productivity mg/(m2·d) (according to carbon) MLD mixed layer depth m SST sea surface temperature °C SSS sea surface salinity MAT mesopelagic average temperature °C MAS mesopelagic average salinity MAO mesopelagic average oxygen concentration μmol/kg
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