Monthly Kachemak Bay Report, August 2026

Summary

August 2026 had the highest average global sea surface temperatures on record. Bucking this situation, most Kachemak Bay waters remained colder than normal, by over 3 standard deviations in some places. Surface salinity was below, but the remaining of the water column had above normal salinity values, resulting in a water column that was substantially more stratified than normal for the season. Chlorophyll concentrations were highest in the outer bay, but were below normal at most stations.

A complete set of up-to-date graphs can be found on this Google Drive: section plots and time sections (request access from martin.renner@noaa.gov).

Conditions

After a cold June-July, air temperatures leading up to this survey had returned to normal (Figure 1). After a wet July, precipitation amounts were close to normal in the weeks before this survey. Results are shown for the Kasitsna Bay Lab oceanographic survey conducted on 26 August 2026 by Hans, Kris and Paul in calm conditions.

Physical oceanography

Temperature

Water temperatures during the August survey ranged from a minimum of 8.3 °C at station AlongBay-5 (165 m depth), in the outer bay, to a maximum of 12.9 °C at station AlongBay-13 (1 m depth) in the inner bay. Strong vertical stratification was observed throughout the bay, with a shallow layer of warmer, fresher surface water over cooler, more saline waters below (Figure 2 (a), top-left panel).

Surface waters remained warmer in the inner than in the outer bay, however subsurface layers beyond 20 m were warmer in the outer than in the inner bay. Satellite sea surface temperature data
showed with a cooler pool of 8-9 °C waters remaining off the head of the bay in lower Cook Inlet (Figure 3).

(a) AlongBay Transect from Kachemak Bay entrance (left) to Bear Cove (right).

Figure 3: Sea surface temperature in the Gulf of Alaska, based on satellite remote sensing data

Figure 4: Surface water temperatures (0-20 m) at station T9-6, off the Homer Spit, compared to the seasonal means

Throughout the bay, subsurface temperatures were colder than normal, in some areas by a remarkable 3 standard deviations (Figure 2), a pattern that has persisted since last December (Figure 6). Surface waters down to about 5 m, however were warmer than normal in many places, especially in the inner bay, reaching 2 standard deviations in the most extreme case (Figure 2 (a), top-right panel). Homer harbor and mid-bay station T9-6 registered below-normal temperatures (Figure 6, Figure 7).

For the first half of this year, surface waters of the Gulf of Alaska were colder than normal. This was no longer the case, with only small pockets of water with negative temperature anomaly remaining in Cook Inlet and surrounding waters, whereas most of the North Pacific was, in some areas dramatically, warmer than normal, as shown by satellite sea surface temperature data (Figure 8). Incidentally, global sea surface temperatures in August 2026 were the highest ever recorded.

Figure 5: Water temperature at the Homer Harbor SWMP station, operated by the KBNERR, at approximately 7 m depth (This is the deeper of two sensors, but is still located in the surface layer).

Figure 6: Recent bottom water temperatures in outer Kachemak Bay (station AlongBay-5, 172 m depth) compared to the long-term mean at this location.

Figure 8: Global sea surface temperature anomaly, based on satellite remote sensing.

Salinity

Salinity values during the July survey ranged from 18.8 PSU at station AlongBay-11 (1m depth) in the inner bay to 31.4 PSU at station AlongBay-2 (115m depth) in the outer bay. The water column was strongly stratified, with fresher waters in a 3-10 m surface layer (Figure 2).

Almost the entire water body of Kachemak Bay was saltier than normal, the exception being the thin surface layer which was fresher (but also thinner) than normal (Figure 2, Figure 7, Figure 9).

Figure 9: Variability of deep-water salinity at station T9-6 off the Homer Spit, compared to the seasonal means.

Turbidity

Most of the bay was less turbid than normal, but no consistent gradients were apparent (Figure 2).

Biological conditions

Phytoplankton

Elevated chlorophyll concentrations were found in the upper 20 meters throughout Kachemak Bay, with the highest values at station 9-9 (1m depth) close to the Homer Spit (Figure 2 (a)). Integrated over the entire water column, chlorophyll concentrations were slightly higher in August than in July, as expected. Concentrations were still below the long-term mean, however (Figure 10). Our partners from the Kachemak Bay National Estuarine Research Reserve detected a bloom of potentially harmful Dinophysis spp. off Gull Island in the inner bay, Pseudo-nitzschia spp. at “abundant” levels in several outer bay samples, and Alexandrium spp. as “present” throughout the bay (Kim Schuster, Kachemak Bay NERR, The Phyto Times, 27 August 2026). Last year, Alexandrium spp. bloomed in Kachemak Bay. A trigger for this may have been the much earlier onset of warm bottom waters (Figure 11). If there are cyst beds of Alexandrium in Kachemak Bay, these would have had several weeks of time to germinate and grow in 2025. By contract, in 2026, the a threshold temperature of 8 °C was reached a month later, for T9-6 the latest date we ever recorded.

Figure 10: Seasonal patterns of chlorophyll concentrations at outer-bay station AlongBay-5 (integrated over the full water column).)

Figure 11: Interpolated date when a threshold temperature of 8 °C was reached in bottom waters at outer bay station AlongBay-5, midbay station T9-6, and inner bay station AlongBay-7. Before this temperature is reached, local germination of *Alexandrium* spp. cysts is unlikely.

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