Underwater optical environment in the coastal waters of British Columbia, Canada

Introduction

Materials and methods

Study area

The SoG exhibits estuarine circulation characterized by lower salinity seaward surface flow driven by the discharge of the Fraser and other rivers, and a deep return flow of more saline and nutrient-rich waters from the Pacific Ocean into the Strait (Li et al. 2000). Vigorous tidal mixing occurs in Haro Strait and Boundary Pass (Fig. 1), where deeper waters from the Pacific Ocean are mixed with surface waters (Masson and Cummins 2004). The waters of the SoG within the euphotic zone are influenced by the discharge of the Fraser River to different degrees depending on the season, surface currents, and tides. Thus, the salinity structure in the SoG creates the stratification that is always present in these waters (Tully and Dodimead 1957; Waldichuk 1957).

Fig. 1.

High concentrations of suspended particles and dissolved organic matter in the surface layer are generally observed, particularly in the Fraser River plume (Pharo and Barnes 1976; Kostaschuk and Luternauer 1989; Kostaschuk et al. 1993; Luternauer et al. 1998; Kostaschuk 2002). Primary productivity is high (∼280 gC·m⁻²·year⁻¹) and mainly limited by light, as for most of the year nutrients are supplied in excess by inflow of upwelled water from the Pacific Ocean (Thomas and Grill 1977; Stockner et al. 1979; Harrison et al. 1983, 1994; St. John and Pond 1992; Yin et al. 1995, 1997; Mackas and Harrison 1997; Yin and Harrison 2000).

In the SoG, during the spring and summer, waters with the highest attenuation (e.g., beam attenuation coefficient of blue light at 411 nm, c_t′(z,411) ≈ 8.0 m⁻¹) and lowest ratio of absorption to scattering (a_t′(z,411)/b_p′(z,411) ≈ 0.4) are mostly related to the Fraser River plume, due to the high particle concentrations. The Fraser River plume water was defined as optical water mass 1 (OM1) by Loos and Costa (2010). Northern Strait surface waters and waters below the optical attenuation cline, OM2, are defined as transitional optical waters, and are characterized by lower attenuation (c_t′(z,411) ≈ 1.2 m⁻¹) and higher IOP ratios (a_t′(z,411)/b_p′(0−,411) ≈ 1.0). Finally, deeper waters (z ≥ 10.0 m), OM3, generally exhibited the lowest attenuation (c_t′(z,411) ≈ 0.5 m⁻¹) and highest absorption to scattering ratios (a_t′(z,411)/b_p′(z,411) ≈ 2.0), indicating that absorption by CDOM is the dominant process (Loos and Costa 2010).

Results and discussion

Optical dynamics

The in situ and modelled optical data allowed for a description of the light climate in the SoG waters in spring and summer. Generally, the quality and quantity of the in-water light field differed depending on how light was attenuated as a result of different optical constituents.

The reflectance spectra varied in magnitude and shape among OMs and at different times of the year. However, there was a clear transition from waters dominated by the Fraser plume (OM1), through a mixture of plume and Strait waters (OM2), into northern and deeper waters (OM3) (Fig. 2).

Fig. 2.

R_r (z,λ) was highest in OM1 (0.021 sr⁻¹ in April and 0.040 sr⁻¹ in July), and lowest in OM3 (deeper water) (Fig. 2). The strong reflectance in OM1 is a consequence of the high scattering caused by suspended particles and the low absorption-to-scattering ratio for these waters that was observed by Loos and Costa (2010).

The shape of the reflectance spectra varied among water masses and between the spring and summer conditions. There was a clear transition from waters dominated by the Fraser plume (OM1), through a mixture of plume and Strait waters (OM2), into northern and deeper waters (OM3) (Fig. 2). Generally, the reflectance was lowest at 400–450 nm (purple–blue), and peaked at 520–640 nm (yellow–green). The yellow–green peak was less pronounced in OM2 and OM3 than in OM1, consistent with the observation that in OM2 and OM3 absorption by CDOM contributed more to the total attenuation of blue light than did absorption and scattering by suspended particles (Loos and Costa 2010).

The reflectance minimum at 675 nm and small peak at 685 nm (due to absorption and fluorescence of chl a, respectively) were less pronounced in OM1 (Fig. 2). This was a consequence of the absorption by CDOM (1.0 m⁻¹ in April and 0.53 m⁻¹ in July) and increased scattering due to high concentrations of suspended particles (6.9 mg·L⁻¹ in April, 9.9 mg·L⁻¹ in July) in these waters (Loos and Costa 2010), which overwhelmed the effects of chlorophyll.

The high turbidity of the OM1 waters played a strong role in the attenuation coefficient of downwelling irradiance, K _Ed(z,λ), and follows Jerlov’s classification of waters with the highest turbidity. Jerlov’s water body classification scheme is based on the vertical attenuation of downwelling irradiance. In all, there are five typical open ocean spectra (I, IA, IB, II, and III) and nine typical coastal spectra (1–9), with turbidity increasing with class number (Jerlov 1976). In April, in situ K _Ed(z,λ) in OM1 was slightly higher than Jerlov’s most turbid classification for coastal waters (Fig. 3). In July, K _Ed(z,λ) in OM1 was similar to that of Jerlov’s Type 9 in the short wavelengths but exceeded Type 9 above 500 nm. Vertical profiles of E_d (z,PAR)/E_s (0+,PAR) further corroborate the fact that light was attenuated with depth most rapidly in the turbid waters of the Fraser River plume (Fig. 4). The magnitude of K _Ed(z,λ) decreased with distance from the Fraser River; OM3 waters were the clearest of all three OMs, particularly in July, when K _Ed(z,λ) in OM3 was similar to Jerlov Type 1. In April, OM3 waters were somewhere between Jerlov types 3 and 5 (Fig. 3).

Fig. 3.

Fig. 4.

Scalar irradiance and the average cosine

For brevity, optical results will be discussed for four key wavelengths in the blue (411 nm), green (530 nm), and red (650 and 675 nm) parts of the electromagnetic spectrum to show the dynamics of light. HydroLight output predicted all the measured variables with p < 0.05. The p-value was used here simply to demonstrate the significance of the calculated values. In our case, we assumed that p < 0.05 indicated that the modelled results were significant and can be trusted. The relationships were stronger for E_d (z,λ), L_u (z,λ), and K _Ed(z,λ), which are less sensitive to the geometry of the incoming light (Bukata et al. 1995) than for R_r (z,λ) (Table 1).

HydroLight output resulted in low values of $\overline{\mu }$ (z,λ) (∼0.7 at 411 nm) for OM1 waters, indicating a very diffuse light field (Fig. 5), whereas $\overline{\mu }$ (z,λ) was higher in OM2 and OM3 waters. Light was less diffuse away from the plume and in deep water, as indicated by the increase in $\overline{\mu }$ (z,λ) to a maximum of 0.9 at 411 nm in OM3. The high light diffusivity of OM1 is associated with high scattering caused by inorganic suspended particles. The association of in-water light diffusivity with inorganic or organic dominant matter was further explained based on data from Loos and Costa (2010) and according to the ratio of backscattering to particulate scattering, b_b′(z,λ)/b_p′(z,λ), and parameter B of McKee and Cunningham (2005), which considers the modelled backscattering coefficients and in situ particulate scattering coefficients (eq. (3)).

$$B_{\text{wd}}=[b_{b\prime }(z,411)/b_{p\prime }(z,411)]/[b_{b\prime }(z,675)/b_{p\prime }(z,675)]$$

(3)

Fig. 5.

This was demonstrated based on HydroLight-derived backscattering and backscattering ratio (Loos and Costa 2010).

For OM1 waters, b_b′(z,λ)/b_p′(z,λ) values were the highest, decreasing away from the plume and with depth. A high OM1 b_b′/b_p′ ratio (e.g., 0.012 at 530 nm) indicates a high proportion of inorganic particles in the suspended load, because of the high index of refraction of such particles (Twardowski et al. 2001; Boss et al. 2004) and is consistent with the high proportion of inorganic particles in river plume waters (Johannessen et al. 2003). OM2 and OM3 waters had low b_b′/b_p′ indicating a higher proportion of organic particles in these waters (Table 2).

Table 2.

	Optical water mass	411 nm		530 nm		650 nm		675 nm
		April	July	April	July	April	July	April	July
$\overline{\mu }$ (z,λ)	OM1	0.77 ± 0.03	0.67 ± 0.05	0.61 ± 0.03	0.52 ± 0.06	0.62 ± 0.05	0.55 ± 0.09	0.66 ± 0.05	0.57 ± 0.10
	OM2	0.81 ± 0.04	0.82 ± 0.03	0.71 ± 0.06	0.74 ± 0.05	0.78 ± 0.05	0.80 ± 0.05	0.73 ± 0.07	0.75 ± 0.07
	OM3	0.85 ± 0.02	0.90 ± 0.02	0.76 ± 0.04	0.78 ± 0.04	0.84 ± 0.03	0.85 ± 0.03	0.64 ± 0.14	0.63 ± 0.16
Ed (0−,λ) (× 102) (W·m−2·nm−1)	OM1	0.91 ± 1.83	2.19 ± 3.27	8.55 ± 8.00	14.57 ± 11.30	7.00 ± 6.10	10.24 ± 8.13	5.26 ± 5.52	8.28 ± 7.56
	OM2	1.96 ± 3.51	3.44 ± 8.51	17.76 ± 14.04	19.18 ± 23.15	6.75 ± 8.25	8.23 ± 13.96	3.70 ± 5.38	5.75 ± 11.49
	OM3	0.67 ± 2.16	0.02 ± 0.04	7.04 ± 6.30	4.40 ± 3.82	1.44 ± 3.04	0.21 ± 0.32	0.85 ± 2.45	0.06 ± 0.10
Lu (0−,λ) (× 102) (W·m−2·sr−1·nm−1)	OM1	0.002 ± 0.005	0.015 ± 0.024	0.078 ± 0.082	0.25 ± 0.26	0.058 ± 0.066	0.17 ± 0.23	0.037 ± 0.048	0.13 ± 0.18
	OM2	0.003 ± 0.005	0.005 ± 0.011	0.074 ± 0.078	0.059 ± 0.070	0.013 ± 0.0.22	0.013 ± 0.025	0.016 ± 0.020	0.015 ± 0.024
	OM3	0.001 ± 0.002	3.53 × 10−5 ± 5.99 × 10−5	0.023 ± 0.026	0.010 ± 0.007	0.001 ± 0.003	2.19 × 10−4 ± 2.49 × 10−4	0.003 ± 0.005	6.54 × 10−4 ± 6.26 × 10−4
K Ed(0−,λ) (m−1)	OM1	2.28 ± 0.52	2.01 ± 0.67	0.69 ± 0.14	0.84 ± 0.35	0.68 ± 0.06	0.87 ± 0.22	0.87 ± 0.06	1.01 ± 0.19
	OM2	0.75 ± 0.22	0.73 ± 0.27	0.26 ± 0.08	0.26 ± 0.10	0.52 ± 0.07	0.49 ± 0.06	0.69 ± 0.13	0.61 ± 0.09
	OM3	0.48 ± 0.09	0.43 ± 0.10	0.15 ± 0.03	0.15 ± 0.03	0.42 ± 0.02	0.42 ± 0.02	0.48 ± 0.11	0.45 ± 0.10
Rr (0−,λ) (× 103) (sr−1)	OM1	2.00 ± 0.51	6.00 ± 2.00	8.00 ± 2.00	17.00 ± 7.00	7.00 ± 3.00	15.00 ± 10.00	6.00 ± 2.00	13.00 ± 8.00
	OM2	1.58 ± 0.58	1.70 ± 0.60	4.00 ± 1.95	4.00 ± 3.00	1.53 ± 0.83	1.78 ± 1.78	9.00 ± 10.00	8.00 ± 9.00
	OM3	1.40 ± 0.29	1.45 ± 0.42	3.00 ± 1.00	2.00 ± 1.44	1.60 ± 1.15	1.83 ± 1.66	28.00 ± 27.00	29.00 ± 28.00
bb′(z,λ) (m−1)	OM1	0.041 ± 0.015	0.091 ± 0.050	0.038 ± 0.015	0.088 ± 0.052	0.036 ± 0.014	0.083 ± 0.052	0.035 ± 0.015	0.082 ± 0.053
	OM2	0.007 ± 0.005	0.008 ± 0.006	0.008 ± 0.005	0.007 ± 0.006	0.007 ± 0.005	0.007 ± 0.006	0.006 ± 0.005	0.006 ± 0.005
	OM3	0.002 ± 0.001	0.002 ± 0.002	0.003 ± 0.002	0.002 ± 0.001	0.002 ± 0.002	0.002 ± 0.001	0.002 ± 0.001	0.002 ± 0.001
bb′(z,λ)/bp′(z,λ)	OM1	0.010 ± 0.003	0.014 ± 0.006	0.010 ± 0.003	0.014 ± 0.007	0.010 ± 0.003	0.014 ± 0.007	0.010 ± 0.003	0.014 ± 0.007
	OM2	0.009 ± 0.005	0.012 ± 0.006	0.009 ± 0.005	0.012 ± 0.007	0.009 ± 0.005	0.012 ± 0.007	0.009 ± 0.005	0.012 ± 0.007
	OM3	0.008 ± 0.004	0.011 ± 0.007	0.008 ± 0.004	0.011 ± 0.007	0.008 ± 0.004	0.011 ± 0.007	0.008 ± 0.004	0.011 ± 0.007
nEo (0−,λ)	OM1	0.016 ± 0.031	0.029 ± 0.047	0.154 ± 0.141	0.216 ± 0.180	0.159 ± 0.141	0.152 ± 0.130	0.113 ± 0.121	0.117 ± 0.114
	OM2	0.021 ± 0.036	0.031 ± 0.064	0.172 ± 0.111	0.152 ± 0.137	0.071 ± 0.076	0.079 ± 0.110	0.039 ± 0.051	0.055 ± 0.091
	OM3	0.014 ± 0.042	0.000 ± 0.001	0.120 ± 0.106	0.036 ± 0.022	0.029 ± 0.062	0.002 ± 0.003	0.018 ± 0.049	0.001 ± 0.001

Note: $\overline{\mu }$ (z,λ), average cosine of downwelling irradiance; E_d (z,λ), spectral downwelling plane irradiance; L_u (z,λ), spectral upwelling radiance; K _Ed(z,λ) downwelling irradiance attenuation coefficient; R_r (z,λ), underwater radiance reflectance; b_b′(z,λ), particulate backscattering; b_p′(z,λ) particulate scattering; nE_o (z,λ), normalized scalar irradiance; 0–, in the water.

The relationship between B _wd and b_p′(z,675)/a_t′(z,675) indicated that scattering in OM1 was less variable and had lower wavelength dependence (B _wd ≈ 1.0 in April and July) than that in OM2 and OM3 waters due to the higher concentration of suspended particles in OM1. The greater wavelength dependence of the scattering coefficient in OM2 is attributed to the organic nature of the particulate, namely phytoplankton.

Modelled normalized scalar irradiance, nE_o (z,λ), obtained from the ratio E_o (z,λ)/E_o (0+,λ), was the lowest in OM1 and indicated that blue and red wavelengths were quickly attenuated within the first 5 m to below 5% of the surface intensity (Figs. 6a, 6c). Below this depth, available light was predominantly in the green wavelengths (Fig. 6b). This is also supported by the low K _Ed(z,500–600 nm) in the green spectrum (Fig. 4). The modelled attenuation coefficient of scalar irradiance, K _Eo(z,PAR) (0.11–2.90 m⁻¹; Table 3) in spring and summer was similar to values reported by Stockner et al. (1979) and Harrison et al. (1991) in the waters of the SoG during winter and spring.

Fig. 6.

Table 3.

Month	Station	Modelled K Eo(z,PAR) (mean) (m−1)	Modelled K Ed(0−,PAR) (mean) (m−1)
April	S1-1	0.361	0.356
	S1	0.288	0.288
	S2-1	0.169	0.169
	S2-2	0.240	0.245
	S2-3	0.998	0.963
	S3	0.181	0.182
	S3-1	0.260	0.256
	S3-2	0.658	0.653
	S4-1	0.221	0.220
	S4-2	0.337	0.338
	S4-3	0.478	0.491
July	S1-1	0.306	0.308
	S1-2	0.254	0.253
	S1	0.232	0.232
	S2-1	0.286	0.291
	S2-2	0.400	0.400
	S2-3	1.957	1.900
	S3	0.358	0.333
	S3-1	0.563	0.565
	S3-2	0.648	0.618
	S3-3	0.393	0.388
	S4-1	0.211	0.213
	S4-2	0.412	0.415
	S4-3	0.473	0.475
	S5	0.381	0.387
	S5-1	0.264	0.268
	S5-2	0.442	0.441
	S6	0.255	0.257
	S6-1	0.237	0.237
	S6-2	0.283	0.279

Note: K _Eo(z,PAR), scalar irradiance attenuation coefficient; K _Ed(0−,PAR), downwelling irradiance attenuation coefficient; PAR, photosynthetically available radiation, 0–, in the water.

Virtually none of the light that enters the SoG passes all the way through the surface estuarine outflow layer. The depth of separation between the outflowing and inflowing layers has been modelled at 50 m (Pawlowicz et al. 2007) and 30 m (Riche and Pawlowicz 2014), and the depth of the Fraser River plume at 15 m (Johannessen et al. 2006; Masson 2006). The more highly depth-resolved profiles shown here (Figs. 2, 7) suggest that most of the light field is attenuated by the fresh water contained within the uppermost 7 m in spring and summer.

Fig. 7.

By 15 m, only green light remains (Fig. 7), and even then only in areas away from the Fraser River plume (S4, S5, and S6 group of stations in Fig. 1). By 30 m, all of the downwelling irradiance has been absorbed within the water column (Loos and Costa 2010) or reflected back out of it (Figs. 3, 7). Figure 2 shows the high reflectance for the OM1 waters. Consequently, all of the light is attenuated within the outflowing surface layer, which has a residence time of approximately 10 d in the Strait (Pawlowicz et al. 2007). This is consistent with the observation by Pawlowicz et al. (2007) that the SoG is a small net exporter of heat, based on the absorption of shortwave (UV + visible) radiation.

Z _1% was 6.0–22.0 m in spring and 4.0–23.0 m in the summer. The depth of 1% penetration of blue radiation at one of the chlorophyll absorbance peaks (443 nm) was always shallower than 9.0 m and often shallower than 5.0 m. Light at all wavelengths was more rapidly attenuated in the summer than in the spring within the river plume, because of the high scattering by inorganic particles and high absorption by CDOM (Loos and Costa 2010). However, away from the plume, blue wavelengths were more rapidly attenuated in the spring, because of the absorption by phytoplankton (Loos and Costa 2010).

The effects of the attenuation of light can be seen in the depth distribution of phytoplankton, which occurs over the top 30 m in spring, but only over 12 m in summer (Peña et al. 2016). Primary productivity is always limited by light in the southern Strait, so productivity is actually higher in that region in the summer, when more light is available, than in the spring (Peña et al. 2016). In the central Strait, which includes waters under the strong influence of the Fraser plume, phytoplankton are thought to be usually limited by light and occasionally by nutrients (Peña et al. 2016); although, it can be difficult to differentiate the effects of stratification (nutrient limitation) from those of turbidity (light limitation), as the two factors co-occur in the plume. In fact, primary production is low in the plume in summer (Peña et al. 2016), because both light (due to high attenuation and therefore lower E_d (z,λ)) (Fig. 7) and nutrients are in short supply. In the summer, because of their shallow distribution, some phytoplankton are exported from the Strait (Peña et al. 2016) with the illuminated layer of water and absorbed radiation; although, most suspended particles are retained within the Strait (Johannessen et al. 2003).

Conclusions

List of abbreviations

AOP

apparent optical property

CDOM

chromophoric dissolved organic matter

chl a

chlorophyll a

FF

Fournier-Forand

HPLC

high-performance liquid chromatography

IOP

inherent optical property

Minispec OCR

miniature hyperspectral ocean colour radiometer

MSV

Marine Science Vessel

OM

optical water mass

PAR

photosynthetically available radiation

SoG

Strait of Georgia

SD

standard deviation

Fundamental quantities and other symbols

0+ 

above water

0− 

in the water column

B _wd

wavelength-dependence of the backscattering coefficient

p

probability value

r ²

coefficient of determination

λ

wavelength (nm)

z

depth (m)

Z _1%

depth of 1% irradiance (m)

Z _eu

depth of the euphotic zone (m)

Radiometric quantities

L_u

upwelling radiance (W·m⁻²·sr⁻¹)

L_w

water-leaving radiance (W·m⁻²·sr⁻¹)

E_d

in-water downwelling irradiance (W·m⁻²)

E_s

above-water downwelling irradiance (W·m⁻²)

E_u

above-water downwelling irradiance (W·m⁻²)

E_o

scalar irradiance (W·m⁻²)

nE_o

modelled normalized scalar irradiance (W·m⁻²)

Inherent optical properties

a _t′

scattering-corrected absorption coefficient (m⁻¹)

a _c′

scattering-corrected CDOM absorption coefficient (m⁻¹)

b _t′

total scattering coefficient without water scattering coefficient (m⁻¹)

b _p′

particulate scattering coefficient without water scattering coefficient (m⁻¹)

b _b′

modelled particulate backscattering coefficient (m⁻¹)

c _t′

measured total beam attenuation coefficient without water beam attenuation coefficient (m⁻¹)

Apparent optical properties

R_r

radiance reflectance (sr⁻¹)

K _Ed

downwelling irradiance attenuation coefficient (m⁻¹)

K _Eo

scalar irradiance attenuation coefficient (m⁻¹)

$\overline{\mu }$

average cosine

Acknowledgements

References