As Seasons Change, So Does the Pond:
Falling leaves, concord grapes, migrating monarchs, and ripening pumpkins alert us that our New England summer has transitioned into fall. The autumnal cycle brings fewer hours of daylight and cooler temperatures. Exuberant expressions of fall color are daily reminders of this seasonal transition on land. What we may not see is the parallel seasonal cycle taking place in our estuaries.
The decrease in daylight hours and temperatures causes a shift in the biological communities and the physical properties of water. The types of plankton that thrived in warm summer months die off and give way to plankton that flourish in the fall. The amount of oxygen that can physically be dissolved in water increases, as fall temperatures decrease. The amount of dissolved oxygen in the water is key to animal and plant health and it is also an indicator of estuary health.
As fall progresses towards winter, plants and animals in an estuary either die or slow down their metabolic activity. As organisms die or senesce, the raw materials that were incorporated into their bodies, or biomass, are released into the water. One of these raw materials is Organic Nitrogen.
When Nitrogen is incorporated into the biomass of an organism, it can be part of its protein structure (amino acids) or genetic material (DNA and RNA). Microbes (bacteria and archaea) in the water and sediment can transform Organic Nitrogen through biological activity into Inorganic Nitrogen (nitrogen gas, nitrite, nitrate, and ammonium). Nitrogen gas is not harmful to the ecosystem, but an excess of other forms of Inorganic Nitrogen such as nitrate and ammonium can be harmful to the health of our estuaries.
In fact, the accumulation of excess nutrients, or eutrophication, is one of the biggest challenges facing our island ponds. Eutrophication can shift the balance of an ecosystem and cause algal blooms, which if extreme, can lead to hypoxia (depletion of dissolved oxygen) and death of estuarine organisms.