Vertical migration refers to the vertical movement along the water column in response to ontogenic, seasonal and daily changes. The most important type of vertical migration is diel.
Ontogenic Migration: This type of migration is dependent on the organism’s life stage, sex and biological rhythm. Ontogenic migration has only been observed in copepods. Generally, nauplii are found at shallower depths while late nauplii and early copepods are found at deeper depths.
Seasonal Migration: The general pattern of seasonal migration is such that zooplankton are higher up the water column from late winter/early spring to late summer/early fall, and lower in the water column from late summer/early fall to late winter/early spring.
Zooplankton reproduction is timed with the spring bloom. The spring bloom is stimulated by increasing light intensity, warming and stratification of the upper layers and leftover nutrient supplies from the winter. Sometimes eggs or larvae remain dormant throughout the winter and hatch in harmony with the spring bloom in order to gain the maximum benefits of the rich food supply.
Zooplankton tend to remain in deeper layers during the winter to avoid energy expenditure by travelling upwards. Food supply is limited during the fall-winter season because of a decline in primary production, so zooplankton tend to rely mainly on their energy reserves.
Diel Migration (DVM): This type of migration is the most important and common type as it is present throughout the year and in all types of water bodies. The general trend is that zooplankton migrate down during the day and up during the night to feed.
DVM is influenced by structural and dynamic drivers. Structural drivers do not change over a 24 hour period: temperature and food availability. Dynamic drivers do change over a 24 hour period and are influenced by light availability: presence of predators and UV radiation. These drivers interact and influence each other.
One of the earliest proponents for DVM was explained by the damaging effects of sunlight which induces a negative phototaxis. So, there is a clear adaptive advantage to downward migration during the day to reduce exposure to UV radiation. Likewise, zooplankton migrate downwards to avoid visual predators. In lakes where planktivorous fish thrive, the magnitude of DVM is high in Cyclopoid copepods, and Calanoid copepods are found deeper in lakes and ponds during the day when fish are present. However, when food is scarce, zooplankton are less likely to migrate down during the day and risk predation. Thus, starvation induces positive phototaxis.
These drivers are also influenced by water transparency. For example, in conditions where water is very transparent, UV and predator exposure increases. Likewise, water transparency influences temperature gradients as it allows increased sunlight in deeper layers. Increased sunlight also increases primary production and thus, food availability. In this case, food is found at deeper layers and zooplankton will not have to migrate as high up the water column, reducing their exposure to predators and UV and, reducing the distance between food and safety.
UV radiation and visual predators are not the only reasons zooplankton migrate. There are metabolic advantages to diel migration as well. For example, organisms that feed in the warm surface waters at night, can conserve energy by residing in the cooler deeper layers during the day. On the other hand, organisms feeding in the cooler deeper layers during the day, may migrate upwards to surface waters at night where the warmer temperature aid in digestion.
During our study, we had the opportunity to sample a few lakes at night to determine if there was a difference in zooplankton distribution along the water column between day and night. The graphs below show chlorophyll α distribution, which was determined by sampling every vertical 2 meters of the water column via the Van Dorn sampler; and zooplankton distribution, which was determined by the same method via the Schindler trap. Vertical distribution patterns will be discussed.
Chlorphyll α is most abundant in the deeper layers, peaking at 7.5mg/L at 18m in the 25m lake. This does not coincide with the metalimnioc layer which is at about 7.5m. In the remaining layers, chlorophyll α is found at 2.5mg/L. The difference between zooplankton distribution between the day and night samples are notable. There is evidence that zooplankton migrated upwards at night. At a depth of 2m, zooplankton abundance increased from 1/L to 7/L — a seven fold increase, and at 6m, zooplankton increased from 4/L to 12/L. The highest day zooplankton abundance is at 8m at 10.5/L and the highest night zooplankton abundance is at 6m at 12/L. This proves that abundance has increased in shallower depths overnight and that the majority of zooplankton have migrated 2m upwards at night, indicating that vertical migration is occurring.
Chlorphyll α is most abundant in the deeper layers, peaking at 20mg/L at 18m in the 30m lake. In the remaining layers, chlorophyll α is found at approximately 2.5mg/L. The metalimnotic layer is between 10-15m thus, chlorophyll does not coincide with this. There is evidence that zooplankton migrated upwards at night by 4m; the highest abundance is at 10m during the day, and 6m at night, indicating upwards vertical migration. Although some zooplankton are found at shallower depths during the day (1-7/L at 0-6m), at night the total number increases greatly (11-24/L at 0-6m).
Chlorphyll α is most abundant at 20mg/L at a depth of 5m. There is little chlorophyll present at 2m or below 6m, possibly accounting for the lack of day zooplankton below 5m in the Day sample.
Zooplankton abundance is highest at the surface water during the day, peaking at 160/L at 0m. In contrast with the data from Long and Round Lake, zooplankton appears to have migrated 2-3m down during the night. However, night zooplankton abundance is the greatest at 2m below the surface at 140/L. There is likely no positive vertical migration during the night because Elbow Lake is shallow with a maximum depth of only 8.6m. Elbow Lake’s profile is not exceptional from other lake profiles i.e. it has limited visibility (Secchi Disk reading: 3.5m), predators are present and it has epilimniotic, metalimniotic and hypolimniotic layers. What caused the downward migration is unknown but predator avoidance is a possibility–although all three lakes exhibited the same predators. Another possibility, is that since the lake is shallow, UV radiation is consistent throughout the lake during the day, but less radiation might still be present in deeper depths during the night.