0602

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.

Water Clarity, Southern lagoon and St. Lucie river.
The two maps to the right compare water clarity (Secchi Depth) in the St. Lucie Estuary and South Indian River Lagoon for the months of June and Oct 2001. This data is collected by MRC’s eighty five Lagoon Watch Volunteers and four volunteers from the Florida Oceanographic Society. The Secchi Depth measures the penetration of light below the surface of the water using a disk with black and white sections. The disk is lowered into the water until the black can not be distinguished from the white areas due to the lack of light penetration. The larger the Secchi Depth the further the light can penetrate. Light penetration is important because seagrass need light to survive and they grow in the sediment at the bottom of the lagoon. Long periods of low light conditions will kill seagrass on which many fish and mammals depend for food and shelter. In June, most of both forks of the St. Lucie Estuary showed a Secchi depth of less than a meter (~3 ft) and the main channel less than 1.5 meters (4.5 ft). The Sebastian River and the Lagoon in central Brevard were also showing Secchi Depths less than a meter (3 ft) in June. The Comprehensive Plan for the Indian River Lagoon sets a goal of 1.5 meters (4.5 ft) for Secchi Depth.

The map on the lower right shows water clarity had declined by Oct. with the south fork and main channel of the St Lucie showing light was having difficulty penetrating to even a half a meter (1.5ft). Much of the rest of the St Lucie and Indian River Lagoon had Secchi depths below 1 meter (3 ft). Discharges to the St Lucie Estuary from Lake Okeechobee often result in poor water clarity, but discharges from the lake did not occur until Oct 31st. The main cause of the poor water clarity in Sept. and Oct. would be stormwater from residential, commercial and farm lands. The clarity is impacted by fine sediment from lawns, farms and cleared areas getting suspended in the water. Clarity is also impacted by nutrients from fertilizers and septic systems causing algae to grow rapidly and color the water green or brown. When the algae dies it settles to the bottom and combines with fine sediment to create muck. This muck sediment is so fine grained that when wind or a boat wake disturbs it, it stays in suspension clouding the water for up to two weeks.

When we think of fertilizer we think of farms, but homeowners over-fertilizing their lawns can make a huge impact. A natural area will tend to contribute nutrients to a water body as leaves. If a natural area is turned into a farm the amount of nutrients from fertilizers is on average twice the historic amount. If the same natural area is turned into a typical suburban development the quantity of nutrients from over fertilizing of lawns on average is five times the historic amount. Because our home sites are small we can afford to over-fertilize but the sheer number of homes all over-fertilizing can be overwhelming.

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