Recent weather allowing algae to thrive in Lady Bird Lake

Weather Blog

Austin’s summers are characterized by hot temps, plenty of sunshine and high humidity. Although the hot days and warm nights allow us to enjoy activities like float the river, swim in Barton Springs pool and kayak the Highland Lakes… this weather also sets up the perfect breeding ground for harmful algae.

Over the summer, toxins from harmful algae blooms in Lady Bird Lake caused the death of 5 dogs. Consequently, the City of Austin put out a warning to pet owners about keeping their animals out of the lake.

The primary cause for the high levels of harmful algae in Lady Bird Lake is presumed to be this summer’s frequent and intense hot temperatures. With the second hottest August and hottest September on record, the algae flourished with weeks of highs in the 90s/low 100s.

Well now that we’re past the summer months and heading into mid-October… the algae levels should be decreasing. But city leaders announced Wednesday that recent results still show unhealthy levels of toxins in the water. So what’s the deal?

Hot stretches with intermittent cool days

Algae thrives with extended hot temperatures, as this results in warmer water. With no shortage of heat this past summer, the harmful algae was able to grow without limits.

Despite seeing a pair of strong cold fronts in the first couple weeks of October, we have not been able to maintain consistently cool temps. Since October 1st, we’ve seen 12 days of above average temperatures (+82°) and 4 days below average (60s, 70s & low 80s).

Warmer than average days offset cooler than average days

Looking in the near-term, Wednesday’s relatively cool temps in the 70s will be followed by two days of 90s this weekend. In hopes of seeing any sort of drop in algae levels, we need to stay consistently cool… which has not happened… and does not look to happen for at least the next week.

7Day forecast as of Oct 16th: cooler 70s followed by hot days in the 90s

Delay in timing: water vs air

In addition to the forecast, we also need to look at the basic physics. Compared to air, water is less efficient at conducting heat… and therefore, needs more energy to reach a given temperature. Air (a gas) needs less energy to reach a given temperature and therefore, is more efficient at conducting heat.

Bottom line, it takes less energy (or sunlight) to warm up or cool down the air temp… and more to change the temperature of water. There will always be a delay — air temp rises… then the water temp rises… air temp drops… then the water temp drops.

Less mixing = faster/thicker algae blooms

Also, with hotter temperatures and ample days of sunshine, the lake water is resistant to mixing… meaning the warm, algae-ridden layer at the top of the lake will remain in stable condition, not mixing down to the bottom. This would allow higher concentrations of algae in the shallow layers of the lake (where humans/pets interact) while also creating a thriving environment for thicker and faster algae blooms.

Other factors

Other factors that can contribute to high algae growth include high levels of nutrients, increased salinity, changes in rainfall and low water flow.

Flooding last fall has led to an increase in nutrients in local lakes. Also, low rain totals this past July and August in addition to a very quick transition to severe/extreme drought in Travis County only exacerbated the growth the algae this summer and fall.

In conclusion, there are several factors allowing the harmful algae to continue in Lady Bird Lake… but one thing for sure, our hot, dry summer weather in addition to our warmer, inconsistent fall weather isn’t helping in keeping the algae from growing.

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