0603

Winter 2001 - V.16 N.4

THE LAGOON MONITOR

Report from the Indian River lagoon Watch, by Dr Gerry Rosebery and Jim Egan

Water Clarity, Southern lagoon and St. Lucie river.

Dissolved Oxygen - it keeps the fish from suffocating.

Dissolved Oxygen - it keeps the fish from suffocating.

   The maps on the right compare dissolved oxygen (D.O.) in June with Sept. of 2001. Dissolved oxygen is effected by some of the same conditions as water clarity. With warm water temperatures and long sunny days, the lagoon in summer is the perfect habitat to grow seagrass. Historically, the rainfall did not wash much nutrients into the lagoon so it encouraged the growth of plants like seagrass which get their nutrients from soil on the lagoon bottom. Now, with urban and farmlands contributing excess nutrients, species like algae thrive because they can use the nutrients dissolved in the water. Algae not only clouds the water and covers seagrass making it hard for sunlight to reach the seagrass, the algae also impacts oxygen levels. Like all green plants, algae produce oxygen as a by-product of photosynthesis during the day and use oxygen like we do by respiring at night. Every summer day the lagoon oxygen levels increase until dusk when respiration causes the oxygen levels to drop until they hit their lowest level at about dawn. Oxygen dissolved in the water is important because most fish cannot breath from the surface and die of suffocation if the early morning oxygen levels drop much below 2 milligrams of oxygen per liter of water (mgO2/L) which is about 2 parts oxygen for every million parts water.

   The map to the near right shows the early morning dissolved oxygen levels (DO) averaged for the month of June. The areas marked in white had on average poor oxygen levels, and those marked with an X were so poor that fish kills were a likely result. More then half of the fish kills reported in Florida in July of this year occurred in the Indian River Lagoon, mostly in Brevard. Only small segments of the lagoon had "good" oxygen levels over 5 mg O2/l. Typically the lowest levels occurred next to urban areas far from inlets. At inlets, well oxygenated ocean water can help restore oxygen levels. Low oxygen levels killed off desirable fish species or caused them to flee those areas and encouraged species that do not need much oxygen like jellyfish to multiply. The low oxygen areas coincided with the appearance of the Invasive Australian Spotted Jellyfish, which consumed millions of fish eggs in the lagoon and billions last year in the Gulf of Mexico. Over forty dolphins deaths also occurred over the summer in the same area. Since dolphins breathe from the surface, oxygen levels would not affect them but the poor water quality could cause toxic microorganisms to multiply.

   The map on the far right shows that by Sept. of this year, the cooling temperatures and reduced sunlight of early autumn, reduced algae growth and helped stabilize oxygen levels. By Oct. the oxygen levels had recovered to healthy levels throughout most of the Lagoon. Because of shorter days and cooler temperatures we can expect oxygen levels to remain good until the spring brings the warmth and sunlight that algae strive on.

   What can we do to help? Use fertilizers sparingly, plant native plants that don’t need fertilizers, direct runoff from your gutters, driveway and lawn into vegetated areas and not the street where it may end up draining into the Lagoon. Use slow release fertilizers and do not use fertilizers within 50 ft. of any waterbody or ditch. Lawn clippings are rich in nutrients so keep them out of the Lagoon. Its up to us to keep the nutrients out of the water to keep the Lagoon a healthy place for seagrass and fish.