Effect of Artificial Illumination on the Intensity of Nocturnal Vertical Migrations of Amphipods in Lake Baikal

Key words: amphipods, Baikal, daily vertical migrations, artificial illumination, video observations.

Daily vertical migrations (DVMs) of pelagic and even many benthic organisms are characteristic of both marine and continental aquatic ecosystems. The causes of such migrations and factors relevant to them are of great interest of the involved factors is highly interest-ing. For several years, we investigated DVMs of Baikal amphipods (Crustacea, Amphipoda), namely, the noc-turnal migration of many shallow-water benthic species to the pelagic zone. We identified the main life forms of amphipods involved in DVMs, dominant species of the nocturnal migratory complex in several areas of Baikal, and rejected the defense–feeding hypothesis of DVMs traditionally used to explain the vertical migrations of plankton (Takhteev et al., 2000; Mekhanikova and Takhteev, 2001).The prevalence of immature juveniles in the nocturnal migratory aggregations also led to a conclusion that DVMs in most species are not related to reproduction and mating (Govorukhina, 2001). We pro-posed that daily migrations provide for the accumula-tion of a certain sum of temperatures (if a vertical tem-perature gradient is present) or for activation of metab-olism via motion proper in order to complete maturation in due time under conditions of a cold-water lake (Mekhanikova and Takhteev, 2001).

The intensity of DVMs depends on meteorological conditions (first of all, wave height) and illumination on a certain night. The level of illumination is determina-tive in the vertical migrations of Baikal zooplankton (Mogilev, 1955). Previously (Bessolitsyna, 2000), it was noted that the intensity of DVMs of benthic amphi-pods decreased on moonlit nights. In an experiment (Bessolitsyna and Stom, 2001), amphipods making night migrations actively avoided both bright daylight (300–400 lx) and weak artificial light (35–40 lx). On the other hand, we repeatedly observed that a weak arti-ficial illumination (e.g., with a flashlight) attracted amphipods swimming in open water. This fact was used to increase sample size in qualitative collections. At the Baikal Biological Station of the Irkutsk State Univer-sity in Bol’shie Koty, a searchlight installed on the pier had been used for several years to collect amphipods and juvenile sculpin at night. However, it remained unclear if the searchlight attracted nocturnal migrants or just aided in collecting them.

The purpose of this study was to elucidate the effect of weak illumination (in this case, artificial) on the intensity of DVMs. Technical devices for underwater video observations provide new opportunities for inves-tigating the Baikal ecosystem. We have gained the first experience in applying video equipment to studies on DVMs of benthic organisms in Lake Baikal.

Observations were made in the course of expedition aboard the research vessel Professor Treskov (June 2002) at two sites of northern Baikal: in the Solontsovaya Bay on the side of Cape Sagan-Maryan, at the western coast (the area of the Baikal–Lena State Nature Reserve), and in the Peshcherka Bay, on the eastern side of the Bol’shoi Ushkanii Island (figure). The work was done from the vessel anchored in a shoal above a platform.

A Sony TR8000E video camera with an accessory wide-angle lens (to expand the field of view in the aquatic environment) was placed in a sealed box with two 35-V halogen lamps. The signal from the box was transmitted through a cable on deck, where it was recorded by a Hitachi VM-8480LE video camera, with the image being controlled on its color display. Record-ing was made in two modes: under artificial light from the lamps installed on the box and in a night-vision mode, using a built-in infrared emitter of the camera. In both cases, the range of vision was 3–4 m.

Recorded images were analyzed on a wide-screen TV set. Amphipods appearing on the screen were counted using a freeze-frame option when necessary. As some crustaceans could repeatedly enter the field of view, one instance of an animal entering and leaving the frame was recorded as one specimen. Such counting did not allow us to determine the absolute density of crustaceans (per unit water volume) but was appropriate for a compara-tive analysis of their migration activity. Solontsovaya Bay(depth 8 m; June 24, 2002; 1:40−2:25 a.m.). The ground consisted of uneven-sized boulders and pebbles of proalluvial origin, mostly rounded and partly submerged in sand, with single small green sponges.

When the box with lamps onsubmerged, no moving organisms occurred in the field of view, including the period when the bottom was already seen. Before the box reached the bottom, five to eight migrants appeared. Within 10–20 s after the box touched the ground, animal movements became more active: the number of amphipods increased from 2–4 to 10−15 specimens swimming at different distances from the lens. After 1.5–2 min, the number of amphipods in the field decreased again (1–5 ind. per frame). In night-vision mode,no amphipods occurred in the field of vision for 75 s after the box touched the ground for the first time. The box was then transferred to another place, where only a single small amphipod swimming 20 cm above the ground was detected within 25 s. After moving the box along large boulders and placing it on the bottom, amphipods (no more than two specimens per frame) appeared only after 25 s, with the field of view remaining empty most of the time. In a new place, only 11 small amphipods of different spe-cies (precise identification by the recorded image was impossible) appeared before the camera during 160 s. After the next transfer and descent, observations con-tinued for 6 min 55 s, and amphipods (including one egg-bearing female) entered the field of view only six times. No migrants were detected in the course of hoist-ing the box on the deck. Bol’shoi Ushkanii Island(Peshcherka Bay, depth 8 m; June 25, 2002; 2:30–3:10 a.m.). The ground con-sisted of boulders encrusted with sponges on sand with Draparnaldioidesalgae.

When the box submerged with the lamps on, aggre-gations of white swimming amphipods were observed throughout the water column, from the surface to the bottom. They could have been attracted by lights on the deck and on the box, but it appeared that some speci-mens had already been there before the onset of obser-vations. According to our unpublished data, this could have been Micruropus wohliiamphipods, which form mass nocturnal aggregations attracted by the search-lights of vessels in the area of the Ushkanii Islands. The density of migrating amphipods reached a peak at a dis-tance of 2−5 m from the bottom and decreased at greater depths.

Immediately after the box touched the ground and was adjusted horizontally, 50–60 amphipods could be detected in the field of view. They belonged to at least two species: a white one (most probably, Micrurops wohlii) and a dark one. The number of migrants increased by a factor of 1.5 after 30 s and almost doubled after 60 s, with the animals moving closer to the sources of light. A pair of amphipods was noted near the lamps. The density of migrants remained high for 3 min. The white specimens were initially 1.5 times more numerous; later, the proportion of the dark specimens increased, and the ratio of the two forms became equal. The dark amphipods kept closer to the bottom than the white ones: when the lens was directed upward, the latter began to prevail, with the number of migrants in the field of view reaching 180 specimens. When the lens was pointed to an area that had not been illuminated, the recorded density of aggregation proved to be 13 times lower. This fact indicated that artifi-cial light attracted the amphipods. In the course of lifting the box, the highest density of “light” migrants (about 60 specimens in the field of view) was observed at depths of 0.5–3 and 5–7 m. In the night-vision mode,virtually no migrants were detected. In the bottom layer (when boulders appeared), no more than 50 specimens were found in the field of view. When the box touched the ground, they disappeared for a minute because of roiling and then started appearing again, in one or two specimens. The same was observed in a different location. When the box was lifted to the pelagic zone, only three spec-imens appeared on the screen during 20 s. After return-ing the box to the bottom for 4 min 15 s, the migrants were detected in the field of view only twice. Thus, observations at both sites showed that light from the lamps of the video box obviously stimulated the movement of amphipods in the open water of the lit-toral zone. Weak artificial light attracted these animals and activated their migratory behavior. Therefore, the observed decrease in the intensity of DVMs on moonlit nights (Takhteev and Bessolitsyna, 1999; Bessolitsyna, 2000) is probably explained by some as yet unknown factors, rather than by avoidance of moonlight. Video recording does not allow accurate species identification in most cases. It may well be that different species of amphipods respond to artificial light in different ways and the picture observed in this study is averaged. Fur-ther investigations are necessary for elucidating the responses to artificial illumination in different species and life forms of amphipods.

Video observations confirm our earlier conclusion (Govorukhina, 2001) that DVMs of amphipods are not related to mating: in this study, only one pair swimming in the pelagic zone was noted.

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