(a) What factor controls the productivity of open-water ecosystems? (b) Distinguish between hypolimnion and epilimnion.

In open-water ecosystems, the primary factor that controls productivity is the availability of nutrients, mainly nitrogen and phosphorous. These nutrients stimulate the growth of phytoplankton, which are the base of the aquatic food web in an open-water ecosystem. Sunlight is also a crucial part of productivity but often varies less than nutrient availability.

Hypolimnion is the lower and colder layer of water in a thermally stratified lake or reservoir. The bottom layer of the water body has a temperature of about 4 degrees Celsius, and it remains isolated from the upper layers due to temperature differences. This causes limited mixing between these layers, and the hypolimnion tends to have less dissolved oxygen than the surface layers, which can lead to a build-up of nutrients and decomposition products.

Epilimnion, on the other hand, is the upper and warmer layer of water in a thermally stratified lake or reservoir. This layer is well-mixed due to wind and solar heating and has more dissolved oxygen as it is in direct contact with the atmosphere. It is also where most of the biological activity, such as photosynthesis, takes place as it has more access to sunlight than the hypolimnion.

To distinguish between hypolimnion and epilimnion in a thermally stratified lake or reservoir, one can observe the following differences: 1. Temperature: Hypolimnion is colder with temperatures around 4 degrees Celsius, while epilimnion has warmer temperatures that vary based on the season and sunlight. 2. Biological activity: Epilimnion experiences more biological activity due to its proximity to sunlight and higher dissolved oxygen content, while the hypolimnion has limited biological activity. 3. Mixing: Epilimnion is well-mixed due to wind and solar heating, while the hypolimnion experiences limited mixing due to the temperature differences between the layers. 4. Dissolved oxygen: Epilimnion has a higher dissolved oxygen content, while hypolimnion has lower dissolved oxygen levels due to limited contact with the atmosphere and lesser mixing. 5. Nutrient accumulation: Hypolimnion often accumulates nutrients and decomposition products due to less mixing and limited exchange with the upper layers, while epilimnion has a more balanced nutrient distribution due to its mixing and biological activities.