Increased snow cover on a water body can lead to a sudden spurt in phytoplankton population which are not dependent on photosynthesis. This can have both positive and negative effects on the ecosystem.

Improved Oxygen Exchange and Nutrient Recycling in the Lake:
- Phytoplankton are microscopic plants that form the foundation of the aquatic food chain.
- They undergo photosynthesis, utilizing sunlight, carbon dioxide, and nutrients to produce oxygen and organic matter.
- In winter, when the water body is covered with snow, sunlight penetration is reduced, limiting photosynthesis for most phytoplankton.
- However, certain phytoplankton species are mixotrophic, meaning they can obtain energy through both photosynthesis and by consuming other organisms.
- These mixotrophic phytoplankton can take advantage of the increased availability of organic matter in the water due to decaying plant material and dead organisms.
- They can rapidly reproduce and thrive under these conditions, leading to a sudden spurt in their population.
- As they grow and reproduce, they release oxygen into the water, improving oxygen exchange and promoting nutrient recycling.
- The increased oxygen levels benefit other organisms in the ecosystem, such as fish and invertebrates, which require oxygen for survival.

Condition of Winterkill Causing Large Scale Death of Fishes and Organisms:
- However, the sudden increase in phytoplankton population can have negative consequences as well.
- When phytoplankton die and decompose, they consume oxygen during the decomposition process.
- If the rate of decomposition exceeds the rate of oxygen production, it can lead to a condition known as winterkill.
- Winterkill occurs when oxygen levels in the water become critically low, causing large-scale death of fishes and other organisms.
- The sudden spurt in phytoplankton population can contribute to winterkill by depleting oxygen levels in the water.
- Additionally, the decomposition of the excessive phytoplankton biomass can also release toxins, further exacerbating the negative impacts on the ecosystem.

In conclusion, increased snow cover on a water body can result in a sudden spurt in phytoplankton population, particularly mixotrophic species. This can lead to improved oxygen exchange and nutrient recycling in the lake, benefiting the ecosystem. However, it can also contribute to winterkill, causing large-scale death of fishes and other organisms due to oxygen depletion.