During 1998–1999, the Laptev Sea was covered with sea-ice from the beginning of October 1998 to the end of July 1999 (Figures 5–7, 8A, 9A). According to daily ice concentration data, freeze-up began on 25 September 1998 (Figures 5A, 8A, 9A) and was persistent at Lena and Yana from 5 October onward (Dmitrenko et al., 2002). Sea-ice concentration from both PIOMAS and passive microwave NASA data reached 90% on 8 and 9 October 1998 for Yana and Lena, respectively (Figures 8A, 9A). PIOMAS estimates of landfast sea-ice thickness at Lena increased from ∼0.5 m on 15 October 1998 to 2.15 m on 30 April 1999 (Figure 9A), which corroborates our in situ observations from May 1999 and generally agrees with the latest satellite altimetry data from the area (e.g., Perovich et al., 2020). At Yana (Figure 8A) ice thickness increased from ∼0.65 m on 15 October 1998 (Figure 5B) to 2.6 m on 18 April 1999 (Figure 6B), which significantly exceeds ice thickness observations of the mobile pack ice in the Laptev Sea from the latest satellite and mooring observations (Hendricks et al., 2018; Belter et al., 2020). Overall, it seems that PIOMAS accurately represents landfast ice thickness but overestimates pack ice thickness in the eastern Laptev Sea.

The location and extent of the polynya varied throughout winter 1998–1999 according to the surface winds. Using the time series of winds and RADARSAT-1 imagery we provide the context on the ice cover relative to the mooring locations. South-westerly winds up to 8–9 m s^–1 from 16 to 20 December 1998 (Figure 4A) led to the formation of a polynya along the landfast ice edge that bisected the area between Lena and Yana (Figure 5C). Between December and February the landfast ice edge extended offshore, and reached the location of Yana. During February 1999 the coastal polynya occurred only in a chain of separate, relatively narrow flaw leads that extended over Yana, as evident on 22 February 1999 (Figure 5D). By March 26, the landfast ice extended ∼2–3 km beyond Yana (Figure 1B), where it remained stable until its collapse in mid-May (Figure 6D). However, from mid-March to mid-May there were several large polynya events along the landfast ice edge (Figures 5, 6) that were driven by southeasterly winds (Figure 4A). These events led to considerable new ice growth within the polynya under cold air temperatures (Figure 4B). Between 24 and 27 March, southeasterly winds up to ∼8 m s^–1 created a large polynya (Figures 4A, 6A). In situ data from Kotelny Island reveal that southeasterly winds peaked at 16 m s^–1 during this period (Dmitrenko et al., 2005b). Following this event, winds decreased to 3–5 m s^–1 during the first week of April, although they remained from the southeast and maintained a polynya around 10–13 km wide through to April 5 (Figure 6A). Strong southeasterly winds up to 9 m s^–1 returned on 12 April, and gradually turned to a westerly heading around 14 April, which led to an expansion of the polynya by 18 April (Figure 6B). The largest polynya opening occurred in early May, when the southeasterly winds from 3 to 5 May (Figure 4A) caused the polynya to extend up to 130 km from the landfast ice edge and ∼70–80 km from Yana, which was still located under the landfast ice (Figure 6C). The opening of the polynya generated widespread formation of young ice between late March and early May, during a period of overall low air temperatures (Figure 4B) and subsequently high ice formation rates.

The landfast ice edge collapsed prior to 13 May, and was likely the result of strong southeasterly winds from 9 to 12 May (Figures 4A, 6D). This caused Yana to be covered by the polynya where pieces of broken off landfast ice were still located. This is also confirmed by the sea-ice concentrations from both PIOMAS and NASA (Figure 8A). Consequently, the landfast ice edge stabilized about 15 km east of Yana until the end of June 1999 (Figure 7).

In contrast to Yana, Lena was located beneath the landfast ice during the entire ice-covered period until the end of June when the landfast ice over the southern Laptev Sea started to breakup (Figure 7B). However, both the PIOMAS and NASA ice concentration data show a drop in sea ice concentration to 40% at Lena between 5 and 30 June (Figure 9A). This is an obvious error, and was likely caused by the onset of surface melt and presence of melt ponds on top of the landfast ice during June. In light of this error, the abrupt increase in under ice illuminance from ∼10² to 10⁴ lux between 5 and 30 June (Figure 9B) seems to be artificial.

The CTD profiles collected during mooring deployment (August 1998) and recovery (September 1999), as well as in April and May 1999 at Lena and Yana show significant (i) spatial, (ii) seasonal, and (iii) interannual differences in the salinity profiles.

The zooplankton communities of the Laptev Sea shelf have a low diversity, and are composed mostly of copepods, with a domination of small-sized organisms highly tolerant to low salinity. The structure of the zooplankton communities in different regions of the Laptev Sea shelf strongly depends on the spatial distribution of the riverine water (Abramova and Tuschling, 2005). Zooplankton communities of the coastal zone consist of freshwater and brackish-water species in summer, and brackish-water species in winter. In the eastern and southeastern Laptev Sea, where the surface salinity during summer gradually increases northward from ∼10 to 22 (Dmitrenko et al., 2010b), the zooplankton assemblages consist of brackish–water neritic species. In contrast, the western and northeastern Laptev Sea affected by the oceanic water is inhabited by marine neritic fauna (Kosobokova et al., 1998). Finally, the central Laptev Sea shelf is characterized by the transitional brackish–marine zooplankton species assemblage (Lischka et al., 2001; Abramova and Tuschling, 2005).

The brackish-water copepods Drepanopus bungei, Limnocalanus macrurus, and Pseudocalanus major are the major contributors to the zooplankton abundance and biomass in the coastal regions of the Laptev Sea (Abramova and Tuschling, 2005). In the eastern and southeastern coastal zone, the brackish-water copepods D. bungei, P. major, L. macrurus, and Acartia longiremis dominate during summer (Lischka et al., 2001; Abramova and Tuschling, 2005). The overall zooplankton abundance strongly depends on population of the copepod D. bungei (Abramova and Tuschling, 2005). The seasonal increase of its total abundance from winter to late summer ranges from 30- to 60-fold, while the interannual variations were only two-fold on average (Abramova and Tuschling, 2005). A transitional community with both the brackish-water and neritic marine components (20% and 15% of the total abundance, respectively) occupies the central Laptev Sea shelf (Abramova and Tuschling, 2005). Marine copepods Calanus glacialis, Metridia longa, Pseudocalanus acuspes, and Oithona similis are abundant in summer, with the first three species dominating the zooplankton biomass. The copepod nauplii constitute up to 48% during summer (Abramova and Tuschling, 2005). The range of interannual variability of zooplankton abundance in this region was assessed based on sampling in August 1998 and 1999 and in April–May 1999. In 1998, the total abundance was consistently higher compared to 1999 over the entire sampling area in the eastern Laptev Sea. Moreover, the regional patterns of zooplankton distribution were correlated with the riverine water spatial distribution (Abramova and Tuschling, 2005). Over the western and northeastern Laptev shelf, marine and neritic copepods dominate with Calanus spp., O. similis, and Pseudocalanus minutus comprising ∼44% of the total abundance. In the northeastern region, the copepod A. longiremis was also recorded (Abramova and Tuschling, 2005). The populations of large Arctic copepods M. longa and Calanus hyperboreus in the northern Laptev Sea seems to be mainly supplied by advection of Atlantic and Arctic water. Contrary, C. glacialis is known to maintain its local population over the central and northern Laptev Sea shelf with the highest reproductive activity in the ice marginal zone over the continental slope (Kosobokova and Hirche, 2001; Ershova et al., 2021).

A mosaic pattern of the zooplankton distribution along with strong regional and seasonal variability of abundance and biomass is typical for the Laptev Sea plankton ecosystem. Recent biomass assessments were presented by Kosobokova et al. (1998), Abramova (1999), and Lischka et al. (2001). During summer 1993, for the domain of our interest in the eastern Laptev Sea shelf, the maximum zooplankton biomass was found near the Lena Delta (104 mg DM m^–3; Fahl et al., 2001). In the southeastern, central, western and northwestern Laptev Sea shelf, biomass values ranged from 8 to 119 mg DM m^–3 demonstrating pronounced patchiness (Lischka et al., 2001). During fall 1995, maximum biomass was found close to the Lena Delta (263 mg DM m^–3). In the coastal zone, biomass was very low, similar to the summer 1993, ranging from 0.3 to 31 mg DM m^–3. Data from 1993 to 1995 indicate that there is enhanced production of zooplankton close to the river outlets. It was suggested that the extremely high zooplankton abundance and biomass in this area was related to dissolved and particulate matter input with the riverine waters resulting in enhanced phytoplankton production close to the river mouths (Lischka et al., 2001).

In general, the MVBS diurnal signal follows the seasonal variability of the sun illuminance except for the period of polar day and polar night when the sun is above and below the horizon 24 h a day, respectively. In Figures 8B, 9B, blue and red arrows depict the periods of polar day and polar night, respectively. During polar day and polar night, the diurnal pattern of MVBS completely vanishes through the water column resolved with ADCP data at both Yana and Lena (Figures 8C–G, 9C–G, respectively). During the transitional period between polar night and polar day (black arrows Figures 8B, 9B), the diurnal cycle in the sun illuminance is mimicked by a daily cycle in MVBS down to a depth of 26 m at Yana and 15.5 m at Lena. Below 15.5 m at Lena, the diurnal patterns of MVBS are not present (Figure 9G).

During the fall transition from polar day to polar night (approximately the end of August and beginning of September 1998 for Lena and Yana, respectively), the occurrence of the MVBS diurnal pattern corresponds to a decrease in the midnight surface layer illuminance below 1 lux (Figures 8D–G, 9D–G), the threshold, which is associated with illuminance during the deep twilight. Further onward, until the beginning of the polar night, the MVBS diurnal signal roughly follows the 1 lux illuminance threshold with acoustic backscatter enhanced during the dark time once illuminance falls below 1 lux. Once polar night begins and the under ice-illuminance falls below 1 lux for 24 h a day, the MVBS diurnal pattern vanishes.

The MVBS diurnal pattern resumed during the spring transition from polar night to polar day (approximately the end of January and beginning of February 1999 for Lena and Yana, respectively) (Figures 8D–G, 9D–G). This corresponds to an increase in the under-ice illuminance to >1 lux at noon (Figures 8B, 9B). At Lena, the MVBS diurnal signal followed the 1 lux threshold and vanished once the under-ice illuminance exceeded the threshold throughout the day shortly before the start of polar day at the beginning of May 1999 (Figures 9B–G). During this time, Lena was still located beneath the landfast ice that was ∼2 m thick (Figures 6C,D, 7, 9A).

In contrast, at Yana, the MVBS diurnal pattern completely vanished at shallower depths (10 and 14 m depth; Figures 8C,D) and was strongly disrupted at lower depths (18–26 m depth; Figures 8E–G) around the end of March, approximately 1-month prior to the start of polar day when the MVBS would be expected to end. The timing of this corresponds to the formation of the coastal polynya ∼2–3 km from Yana between 24 and 27 March 2007 (Figures 1A, 6A–C). By 29 April 1999, when the midnight under-ice illuminance first exceeded 1 lux (Figure 8B), the polynya extended ∼65 km eastward from Yana with open water extending approximately 15 km off the landfast ice edge (not shown). During this time, low MVBS (<−80 dB) was observed at 10 and 14 m depth throughout the day, with no diurnal cycle (Figures 8C,D). Low MVBS during April 1999 also confirms that Yana was still located beneath the landfast ice cover, otherwise, an enhanced acoustic backscatter would be recorded due to frazil ice generated by new ice growth within the polynya during a period when air temperatures remained relatively cold (−19°C; Figure 4B; Dmitrenko et al., 2010a). Note that the MVBS diurnal pattern at Lena located about 120 km from the landfast ice edge did not deviate until the very beginning of May 1999 when under-ice illuminance exceeded 1 lux (Figure 9).

Finally, actograms in Figures 8C–G, 9C–G suggest no deviation of the MVBS diurnal pattern that can be associated with moonlight. During full moons, the midnight under-ice illuminance increased to ∼0.001 lux in February–March 1999 and up to ∼0.01 lux in October 1998 (Figures 8B, 9B). During fall 1998 and spring 1999, the diurnal signal in MVBS at Yana was rather noisy; however, the consistent deviation of the diurnal pattern in response to the enhanced illuminance during the full moon phase is hardly recognizable (Figures 8B–G). The uncertain data on the total cloud cover from the ERA5 atmospheric reanalysis revealed relatively high cloudiness during full moon events from 33.7% (days 420–430) to 69.5% (days 303–313) (not shown). The moonlight illuminance is reduced by cloud cover from ∼200 × 10^–3 lux to ∼0.2 × 10^–3 lux (Krieg, 2021). We note, however, that DVM can be disrupted when under-ice illuminance is <0.002 lux (Petrusevich et al., 2016). Overall, the combination of highly uncertain cloud cover data from the atmospheric reanalysis with noisy MVBS during polar night makes any suggestions about the impact of moonlight on DVM to be very speculative. At Lena, the relatively stable MVBS diurnal cycle also shows no disruptions that could have been generated by moonlight (Figures 9B–G). During the full moon event in October 1998, the midnight MVBS at 6.5 m depth was reduced (Figures 9B,C), but the mean cloud cover of 38% (not shown) does not allow clear attribution of this DVM disruption to moonlight. However, the moonlight impact on DVM cannot be completely discriminated at Yana and Lena.

During polar day and polar night, the MVBS diurnal signal vanished at both Yana and Lena. During polar day, the acoustic backscatter episodically increased 24 h a day up to −40 to −30 dB showing noisy spikes with a magnitude reducing with depth (Figures 8C–G, 9C–G). In contrast, during polar night, the MVBS signal is generally more stable, though it did begin to decrease around the winter solstice (Figures 8C–G, 9C–G).

At Yana, following the beginning of polar day, the increase in MVBS at 10–14 m occurred after the collapse of the landfast ice edge around 11–12 May 1999. Afterward, Yana was primarily covered by open water (e.g., Figure 7A) or pieces of broken off landfast ice (e.g., Figure 7B). Since the beginning of June 1999, 3 weeks before the summer solstice, the air temperature at Yana exceeded 0°C, and sea-ice started to melt with thickness gradually decreasing from a peak of 1.7 m in April (Figure 8A). The period from the beginning of June to ice-free conditions around 23–27 July 1999 is associated with deepening of the enhanced MVBS signal down to 22 m depth (Figures 8A,C–F). Afterward, and until mooring recovery on 30 August 1999, the maximum MVBS was recorded through the subsurface water layer at 10 m depth (Figure 8C), but MVBS gradually increased at depth over time (Figures 8D–G). During the end of polar day through August–September 1998 (illuminance >1 lux), MVBS increased in the subsurface water layer until freeze-up began on 25 September 1998 (Figures 5A, 8B,C). Overall, MVBS from 10 to 26 m depth in August 1999 exceeded that in August 1998 by ∼10–15 dB at Yana.

During polar night, MVBS at Yana was relatively high prior to the winter solstice (Figures 8C–G). Specifically in the subsurface water layer at 10 m depth, MVBS remained around −55 dB until the winter solstice, at which point it rapidly decreased to about −70 dB, and the acoustic signal became noisy, with random spikes up to −35 dB during January 1999 (Figure 8C). Similar MVBS patterns were recorded at 14–18 m depth, but MVBS values were ∼10–15 dB greater than at 10 m depth (Figures 8D,E).

During polar day at Lena, MVBS in the surface layer (6.5 and 9.5 m depth) began to increase around the time that sea ice concentration began to decrease and modeled under-ice illuminance increased around 5 June 1999 (Figures 9A,C,D). However, from RADARSAT-1 imagery we know that the ice cover remained in place (Figure 7A) and instead melt ponds are suggested to form at the surface. Around this time MVBS increased from <−70 dB to ∼−50 dB at 6.5 m (Figure 9C), followed by an increase in MVBS to −60 dB at 9.5 m approximately 1-week later (Figure 9D). After 6 July, MVBS increased through the entire water column up to a peak of −45 dB at 9.5 m depth (Figures 9C–G). During the end of polar day in August–September 1998 (illuminance >1 lux), increased MVBS was recorded until freeze-up began on 25 August 1998 (Figures 5A 9B,C). Moreover, during August 1998, MVBS exceeded that of August–September 1999 by ∼10–25 dB. For example, at 12.5 and 18.5 m depth, MVBS in August 1999 exceeded that in August 1998 by ∼10–15 dB, with MVBS gradually increasing with depth from about −55 dB at 6.5 m to ∼−40 dB at 18.5 m (Figures 9C,G).

In contrast, during the polar night at Lena, MVBS gradually decreased with depth and decreased over time at all depths. At 12.5 m depth, MVBS decreased shortly after the winter solstice, while at 6.5 and 9.5 m depth, this reduction lagged behind the winter solstice by about 2 weeks (Figures 9C–E). At 15.5 m depth, MVBS rapidly decreased from −40 dB to −53 dB in mid-November (Figure 9F). At 18.5 m depth, a gradual reduction in MVBS was recorded from −35 dB in September 1998 to −60 dB in February 1999 (Figure 9G).

Finally, there are spatial and interannual differences between MVBS at Yana and Lena (Figures 8C–G, 9C–G). In 1998, the polar day period is not sufficiently resolved with mooring observations. However, during August–September 1998, MVBS at Lena exceeded MVBS at Yana by ∼12 dB. In contrast, during the polar day of 1999, MVBS at Yana exceeded that at Lena by ∼15 dB. Moreover, during polar night in 1998–1999, MVBS at Lena exceeded that at Yana by ∼10 dB.

The year-long ADCP time series of MVBS over the eastern Laptev Sea shelf are consistent with seasonal DVM of zooplankton conducted during the transition periods between polar day and polar night (Figures 8, 9). During this time, the MVBS diurnal signal is generated by a diurnal movement of zooplankton toward the surface at dusk and descent before dawn the next morning. It is commonly accepted that DVM demonstrates predator-avoidance behavior (e.g., Hays, 2003). Zooplankton keep away from a relatively well-illuminated surface water layer during the day, reducing light-dependent mortality risk. In contrast, during the polar day and polar night, DVM completely vanished (Figures 8, 9).

The acoustic data from the single-frequency ADCP do not provide any information on the identity of organisms responsible for the observed DVM patterns. Among the zooplankton species abundant in the research area, only late copepodite stages of large-sized copepods Calanus glacialis and Metridia longa (total body length 2.0–5.0 mm) are known to perform DVM in the Laptev and Kara seas (Arashkevich et al., 2018). However, these species migrate to deeper regions from October to March, and were most likely absent from the study area. Small-sized brackish-water copepods Acartia longiremis and Drepanopus bungei (≤1.5 mm total body length) are abundant in the study region year round, although their DVM have not been studied in either the Laptev Sea, or other Siberian shelf seas. This is apparently related to the fact that their DVM in the Arctic, if any, is limited to a few meters (Drits et al., 2017), which is beyond the sensitivity of the conventional zooplankton collection methods. Moreover, from November to March, the size range of these species is below the sensitivity of 300 kHz ADCPs used in this research. The abundant small-sized copepod, Pseudocalanus spp. (predominantly P. minutus and P. acuspes; total body length of copepodite stages IV–V is ∼0.8–1.2 mm), may represent another zooplankton species comprising DVM in the study region. DVM of Pseudocalanus was recorded in the seasonally ice-covered White Sea (Pertsova, 1984), Kara Sea (Drits et al., 2017), and Hudson Bay (Runge and Ingram, 1991). Finally, DVM of two larger crustaceans with body size >2 mm, the amphipods Themisto spp. and mysids, recorded in the Arctic Ocean (e.g., Petryashov, 1989; Fortier et al., 2001), can also comprise DVM in the study region. We assume that copepods Pseudocalanus spp., amphipods Themisto spp., and mysids all may perform seasonal DVM in the eastern Laptev Sea. According to their size range, they can serve as prey for polar cod (e.g., Buckley and Whitehouse, 2017; Cusa et al., 2019; Bouchard and Fortier, 2020; Aune et al., 2021).

In general, DVM at Yana and Lena was controlled by light conditions (Figures 8, 9). Similar to other areas, DVM is triggered by local solar variations, and the timing of migration is sensitive to changes in seasonal day length (e.g., van Haren and Compton, 2013). Our results show that DVM responds to the seasonality of sunlight and the seasonality of sea-ice cover that attenuates light transmission to the water column. DVM at Yana and Lena started to occur once the surface layer midnight illuminance exceeded the threshold of 1 lux. Then DVM ceased entirely once the surface layer illuminance exceeded 1 lux for 24 h a day. These results are consistent with a seasonal cessation of DVM reported by Hobbs et al. (2018) at higher latitudes between 77° and 81° N.

Our results on the 1 lux threshold are consistent with the preferendum (isolume) hypothesis (e.g., Cohen and Forward, 2009). A variant of the preferendum hypothesis, the absolute intensity threshold hypothesis, suggests that an ascent at sunset is initiated once the light intensity decreases below a particular threshold level, and a descent at sunrise occurs when the light intensity increases above the threshold intensity (e.g., Cohen and Forward, 2019). This is in line with our findings on an absolute 1 lux threshold of light, which corresponds to illuminance during the deep twilight. It seems that the particular value of the illuminance threshold depends on the zooplankton species composition comprising DVM. For example, for the eastern Beaufort Sea continental slope, Dmitrenko et al. (2020) revealed an absolute 0.1 lux threshold of light controlling seasonal DVM. This threshold corresponds to moonlight illuminance during the gibbous moon under a clear sky. However, Hobbs et al. (2021) revealed a significantly lower value of light limit of photobehavior for species comprising the backscatter signal (∼10^–7 μmol photons m^–2 s^–1 that is <10^–5 lux; see color scale for under-ice illuminance at the bottom of Figures 8, 9).

While DVM at Yana and Lena was in general controlled by light conditions, during spring 1999 there was a principal difference between the two sites. At Lena, located ∼120 km from the landfast ice edge, DVM was entirely controlled by light conditions (Figure 9). In contrast, at Yana, located near the landfast ice edge, the light-driven behavior of zooplankton was completely disrupted following the polynya opening in March–April 1999 (Figures 8A,C,D). This suggests that DVM was not entirely controlled by illuminance, as there were no abrupt changes to the under-ice light conditions while the polynya was open during March–April 1999 (Figure 8B). Moreover, the acoustic backscatter derived from 2 year-long moorings Khatanga and Anabar deployed in the Laptev Sea polynya area in September 2007–2008 and the short-term moorings deployed along the land-fast ice edge in April 2008 and 2009 also showed no seasonal DVM during spring transition from polar night to polar day (not shown, for mooring locations see Dmitrenko et al., 2012).

We suggest that opening of the polynya near Yana generated far-field effects on zooplankton behavior. Since the opening of the polynya did not cause any change in illuminance at Yana (Figure 8B), the observed interruption of zooplankton DVM at Yana in March–April 1999 could potentially be explained by predator-avoidance behavior. Polar cod (Boreogadus saida) is one of the main predators of the zooplankton taxa likely responsible for the observed DVM (e.g., Buckley and Whitehouse, 2017; Cusa et al., 2019; Bouchard and Fortier, 2020; Aune et al., 2021). Polar cod is known to occupy surface waters over shallow coastal continental shelves in winter and spring (e.g., Gradinger and Bluhm, 2004; David et al., 2016). The zooplankton possibly stopped conducting DVM and remained closer to the seafloor, as a predator-avoidance behavioral response to the increase in polar cod abundance near the sea surface. We speculate on two possible scenarios in which the polynya opening could have increased polar cod abundance. In a first scenario, the opening of the polynya increased primary production in the area, which attracted herbivorous zooplankton, and in turn, polar cod. In a second scenario, polar cod were attracted toward the polynya by the enhanced irradiance, and a domino effect (fish following others in their aggregation) resulted in increased polar cod abundance in an area larger than the polynya, including at Yana ∼2–3 km from the ice edge. Unfortunately, the ADCPs echograms could not detect higher abundance of polar cod near the surface at Yana than at Lena to validate these scenarios because the ADCP signal below the ice is compromised due to strong acoustic reflection from the ice-water interface.

Regardless the mechanisms explaining the stop in DVM upon the opening of the polynya, this study clearly demonstrates that an earlier polynya opening and sea-ice breakup caused by climate change could have strong repercussions on the ecology of Arctic zooplankton. Indeed, if an earlier polynya opening and ice breakup reduce the period during which zooplankton conduct foraging DVM in spring, it also reduces their grazing activity and energy intake. This could result in higher mortality rates and reduced carbon export through the biological carbon pump. Polynya openings could further increase zooplankton mortality by increasing the predation pressure from polar cod because more light from polynya openings increase the prey detection distance of polar cod, improving its capacity to forage on zooplankton (Bouchard and Fortier, 2008; Jönsson et al., 2014; Cusa et al., 2019).

During polar day, when DVM ceased, the suspended particles and frazil ice crystals in the water column enhance acoustic scattering and therefore impact MVBS (Wegner et al., 2005; Dmitrenko et al., 2010a,2020; Petrusevich et al., 2020). Suspended particles and frazil ice crystals return the ADCP acoustic signal, producing enhanced MVBS 24 h a day. For example, Petrusevich et al. (2020) reported enhanced MVBS in Hudson Bay recorded by a 300 kHz ADCP. They attributed this signal to the suspended particles released into the water column during ice melt. Using the same type of ADCP, Dmitrenko et al. (2020) reported a high acoustic scattering attributed to the turbid water layer advected over the eastern Beaufort Sea continental slope. A 300 kHz ADCP moored at the edge of the Laptev Sea coastal polynya showed enhanced acoustical scattering from the frazil ice crystals within the surface mixed layer following the polynya opening in April–May 2008 (Dmitrenko et al., 2010a).

We suggest that at Yana, enhancement of MVBS during May 1999 can be attributed to the frazil ice crystals generated by new ice growth in the polynya following the collapse of the landfast ice edge between 9 and 12 May (Figure 6D). Until the end of May 1999, the air temperature at Yana was below the sea-water freezing temperature (Figure 4B), and formation of frazil ice can occur down to the bottom of the surface mixed water layer. The CTD profile taken below the landfast ice approximately 10 km east of Yana on 6 May 1999 revealed that the surface water layer was mixed down to ∼15.5 m depth (Figure 2B). This depth is expected to increase while approaching the polynya due to intense brine rejection and associated vertical mixing (e.g., Dmitrenko et al., 2005b; Kirillov et al., 2013). From the beginning of June through to July 1999, the noise-type MVBS at Yana in Figures 8C–E seemed to be impacted by the release of ice-rafted sediments from the melting sea ice. At Lena, the enhanced MVBS at 6.5 and 9.5 m depth in June 1999 (Figures 9C,D) was likely associated with release of sediments from the sediment-laden landfast ice. The formation of the sediment-laden sea-ice presumably occurred during fall and early winter due to suspension freezing in the coastal polynya (e.g., Ito et al., 2019; Barber et al., 2021). At Lena, the artificial reduction of sea-ice concentration in June 1999 (Figure 9A) was attributed to the formation of melt ponds on top of the landfast ice that confirms sea-ice melt occurred during June.

In contrast to MVBS in June, the enhanced MVBS at both Yana and Lena in July and August 1999 seem to be attributed to intrusions of turbid water associated with river runoff. The Lena River plume is enriched with suspended sediments (e.g., Pivovarov et al., 1999; Wegner et al., 2005). Based on observations from an oceanographic survey in May–June 1996, Pivovarov et al. (1999) revealed that the river runoff water enriched with suspended sediments was laterally advected >80 km off the Lena Delta in 1 week following spring breakup. Based on this result, we suggest that intrusions of turbid riverine water generated high MVBS at Lena and Yana in July–August 1999 (Figures 8C–G, 9C–G).

The interannual variability of MVBS during polar day also suggests the role of riverine turbid water in generating enhanced acoustic scattering. At Yana, MVBS in August 1999 exceeded that of August 1998 by ∼10 dB (Figures 8C–G). In contrast, at Lena, the MVBS pattern was opposite, with MVBS in August 1998 exceeding that in August 1999 by ∼5 dB at 6.5 m (Figure 9C) to ∼15 dB at 18.5 m (Figure 9G). This interannual variability is consistent with the interannual variability in wind forcing between these two summers (Figure 3). In summer 1998, northerly winds diverted the riverine water enriched with suspended sediments onshore, generating a positive salinity anomaly at Yana and negative salinity anomaly at Lena (Figures 2A, 3), and increasing the acoustic backscatter at Lena during August 1998 (Figures 9C–G). Conversely, in summer 1999, easterly winds caused a negative salinity anomaly at Yana due to the northward advection of riverine waters, which increased salinity at Lena (Figures 2B, 3) and enhanced acoustic backscatter at Yana in August 1998. Overall, this finding highlights the role of wind forcing in dictating the distribution of the river plume and in turn driving salinity anomalies and generating acoustic scattering over the eastern Siberian shelf (e.g., Dmitrenko et al., 2005a,2008, 2010b). In the cyclonic regime (e.g., summer 1998) the southeastward diversion of the Laptev Sea riverine water results in a negative salinity anomaly to the east of the Lena Delta and farther to the East Siberian Sea, and a positive anomaly to the north of the Lena Delta. Anticyclonic wind forcing (e.g., summer 1999) results in negative salinity anomalies northward from the Lena Delta due to freshwater advection toward the north, and a corresponding salinity increase southeastward (Dmitrenko et al., 2005a).