In a typical estuary you would find brackish water conditions throughout the system.  The lower salinities (say 0-10 parts/thousand – ppt) would be found at the head of the bay – near the point where the river(s) discharge.  Near the mouth – where the Gulf (ocean) enters the bay – it could be between 20-30 ppt.  The Gulf of Mexico itself is usually around 35 ppt.  When high tide comes, particularly spring high tides, you can find higher salinities throughout the bay.  The opposite is true for low tides.  All estuarine organisms are adapted to deal with some change in salinity throughout the day.  Some prefer the lower salinities and are found near the head of the bay, others prefer higher salinities are found near the mouth, and some can tolerate almost anything and could be found anywhere in the bay.

The salinities in lower Pensacola Bay – Big Lagoon and Santa Rosa Sound – have historically been in the 20-30 ppt range.  Many of the species that live there such as turtle grass, bay scallops, and sea horses need salinity to be within this range.  In recent years the bay area has experienced changes that MIGHT alter the salinity there.

One is increasing rainfall.  The Pensacola Bay area historically received around 60 inches of rain each year – but over the last decade the average annual rainfall has increased to 74 inches.  Two, is increasing development.  New homes, and other structures, are popping everywhere.  With this growth comes more impervious landscape, which increases stormwater run-off into the bay.  With more rain and more runoff, the question we are trying to answer with this project is whether it is enough to alter the salinity to a point where some of the lower estuary species, such as turtle grass and bay scallops, would have problems.

The objective of the salinity monitoring project is to obtain 100 salinity readings from different locations around the bay area to determine what the central tendency of these readings would be; is the lower part of the estuary still between 20-30ppt?

To do this we trained volunteers to obtain a salinity measurement once a week at one of 23 sites around the bay area.  They did this using a refractometer measuring the salinity at the surface near the shore.  To reach the required 100 readings would take an estimated two years and provide a look at what the salinities are at this location over time.

The 2024 3^rd Quarter Update

LOWER ESTUARY (20-30 ppt)

MIDDLE BAY (10-20 ppt)

Lower Bay Discussion

At the end of the 2024 3^rd quarter, 8 of the 13 sites (62%) have reached the 100-reading mark.  Two more will reach this mark by the end of the 4^th quarter.  Of those who have reached the 100-reading mark, 6 (75%) have salinities between the expected 20-30 ppt range.  The two who are not are Lower Perdido Bay and Big Lagoon State Park.  Lower Perdido Bay has historically had a lower salinity than the other lower portions of the Pensacola Bay System and this is not unexpected.  However, Big Lagoon State Park is below the 20 ppt mark.  After 101 readings, the mean salinity was 18 ppt ±7.5.  It ranged from 0-30 ppt with a median value of 19 ppt and the mode was 15 ppt.  14 of the 101 readings were below 10ppt suggesting an impact from stormwater runoff.  The readings were taken at East Beach near the point where the neighborhood of Grande Lagoon discharges into Big Lagoon.  Other than this location, the increase in rainfall does not appear to be impacting the salinities in the lower bay.  The overall salinity for the lower bay is currently at 22ppt.

Middle Bay Discussion

At the end of the 2024 3^rd quarter, only 2 of the 10 sites (20%) have reached the 100-reading mark.  One is above the 90-reading mark, another is at 66-readings, and the remaining 6 sites are below the 50-reading mark.  Of the two above the 100-reading mark, Sanders Beach has a mean salinity of 20ppt – which is at the upper range of what you would expect in this part of the bay.  Bayou Texar has a mean salinity of 14ppt which is what you would expect.  The readings at Hawkshaw number above 50-readings with the measurements at the Memorial site at 91-readings.  Both Hawkshaw sites fall within the 10-20ppt range expected but there is a difference between the Memorial site (where the historic creek discharges into the bay) (MEAN = 13ppt) and the 9^th Avenue site (MEAN = 17ppt) which does not have such.  Whether these differences are significant will be determined when all readings are in.  It is worth noting here that many of these locations show drastic changes during rain events, but the overall salinities do not seem to be altered.  It is also worth noting that many of the middle bay sites have less than 50 readings and confident conclusions are hard to draw.

We will continue to monitor, and the 4^th quarter report will be posted in December.

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