Monthly Kachemak Bay Report, September 2026

Summary

While almost all of the North Pacific is experiencing unusually warm surface conditions, Kachemak Bay was colder than normal, by over 2.5 standard deviations in some places. The water colum remained well stratified, with a fresher surface layer reaching to about 25 m depth. Deep water salinity was about two standard deviations higher than normal. Chlorophyll concentrations were highest near the surface in the far outer bay and near the head of the bay. Those concentrations were high for the season.

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

Air temperatures leading up to this survey were close normal for the season (Figure 1). Precipitation amounts were slightly below-average in the month leading up to this survey. The mountains near the head of the bay had some fresh snow near the peaks. Results are shown for the Kasitsna Bay Lab oceanographic survey conducted on 24 September 2026 by Jen, Kris and Martin in calm conditions.

Physical oceanography

Temperature

Water temperatures during the September survey ranged from a minimum of 8.3 °C at station AlongBay-5 (161 m depth), in the outer bay, to a maximum of 10.5 °C at station AlongBay-11 (1 m depth) in the inner bay. Vertical stratification was observed throughout the bay, with a deepened, over 25 m layer of warmer, fresher surface water over cooler, more saline waters below (Figure 2 (a), top-left panel). In contrast to the rapid verticle changes seen earlier in the summer, the thermocline had a more gradual cooling with depth, which is typical for September.

Satellite sea surface temperature data showed 9-10 °C waters in Kachemak Bay and southeast Cook Inlet (as well as around Kodiak) surrounded by warmer waters in the rest of the inlet and upstream in the Gulf of Alaska (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

The bay was colder than normal at all depths and locations other than near the surface on the south side of the bay. Deep waters were up to a remarkable 2.5 standard deviations colder than normal (Figure 2, top-right panel, Figure 6, Figure 8). Most of Kachemak Bay was about 1 °C colder than normal. This cold spell, driven by a cold winter, has persisted since December 2025 (Figure 4, Figure 5, Figure 6). Surface waters of the Gulf of Alaska, and almost anywhere else in the North Pacific, were warmer than normal in September 2026, as shown by satellite sea temperature anomaly data, making Kachemak Bay a rare exception to the larger scale situation (Figure 7). This reservoir of colder water could help Kachemak Bay buffer some of the effects of expected warming from the developing El Niño.

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

Salinity

Salinity values during the September survey ranged from 26.6 PSU at station 9-8 (1m depth) off the Homer Spit to 31.6 PSU at station AlongBay-5 (161m depth) in the outer bay. The north side of the T9 cross-bay transect had a noticeably thicker layer of fresher, colder surface water, indicative of freshwater outflow from glaciers and streams in inner Kachemak Bay (Figure 2 (b)).

All the deep- and mid-waters of Kachemak Bay were substantially saltier than normal, at depth by as much as two standard deviations (Figure 2, Figure 8). Surface waters in the outer bay were slightly fresher than normal (Figure 2 (a)).

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

Biological conditions

Phytoplankton

Chlorophyll concentrations were highest in the outer bay and near the head of the bay as well as off the Homer Spit (Figure 2). Concentrations in these hotspots were substantially above normal, in places by over two standard deviations (Figure 2 (a)). Surface net tow plankton samples in the outer and inner bay were also extremely dense. By contrast, integrated over the entire water column, chlorophyll concentrations were lower than last month, and also low compared to previous years (Figure 10). Our partners from the Kachemak Bay National Estuarine Research Reserve detected Chaetoceros spp. as the dominant species in all our samples. Potentially harmful taxa Alexandrium spp. and Pseudo-nitschia spp. were detected as “present” in all three samples. (Kim Schuster, Kachemak Bay NERR, The Phyto Times, 1 October 2026).

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

Wildlife

Two orcas were seen off the Homer Spit at T9-6. A red-necked phalarope in the outer bay was late for the season. Notably, no shearwaters were seen, usually common in the bay at this time of year. 3 ancient murrelets in the outer bay were uncommon, but not unexpected for this month.

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