Water Quality: Abiotic, Nutrients and Bacteria in South Slough

Issue Summary:

Estuarine water quality has been generally stable but is trending very slightly downward over the past 7 to 15 years.  Eight of the nine attributes measured show slight downward trends.

Why do we care:

Estuarine water quality affects everything about how estuaries function. Fully functioning estuaries are nursery grounds for about 60% of the commercial and recreational fish and shellfish caught here.  Estuarine waters are the life support system for these animals.

Water and Air Temperatures

Estuary water temperatures are fairly stable over the 7-15 year period covered by the SWMP monitoring stations(Figure 1). The trendlines show a very slight decrease overall for all stations except Sengstacken (slight increase); however, the slopes of all the lines are so slight they are essentially near 0, indicating little change. Air temperature trends from the SWMP weather station (dotted gray) show a slight decrease. There are more extreme air temperature drops in the winters of 2006-2010. Overall, estuaries are predicted to experience an increase in water temperature consistent with ocean water temperatures warming; however, separating changes in water temperature associated with upwelling, longer term climate cycles (Pacific Decadal Oscillation), freshwater temp influences, as well as air temperature may reveal differences associated with location in the estuary, as well as seasonal and annual differences.

Figure 1. Average water and air temperatures in the South Slough estuary

Salinity

Salinity in the South Slough estuary shows a slight increasing trend that is stronger at the riverine sites (WI, SE) (Figure 2). The slopes of the trend lines are very small (0.00006-0.001); however, they are in the positive direction. Estuarine salinity is expected to increase due to climate change mainly through increased seawater intrusion with rising sea levels and salinity will be influenced by changes in precipitation, snow melt, and therefore, freshwater inflows. Location along the estuarine salinity gradient is expected to be an important factor; currently the marine/mid estuarine sites show less change over the monitoring period than the more freshwater sites; however, the marine sites also have less variability than the riverine sites.

Figure 2. Average monthly salinity in the South Slough estuary

Dissolved Oxygen

Estuarine Dissolved Oxygen concentrations show a slight decline over the period 1995-2009 (Figure 3); however, the slopes of the trend lines are very small (range -0.00001 to -0.0002) indicating little change has occurred. Overall, DO concentrations tend to remain above 5 mg/l (good water quality conditions defined by EPA Coastal Condition Report). Oregon DEQ estuary DO criterion are set at 6.5 mg/L. Below 2 mg/l (red dotted line) is the critical point for poor DO conditions (hypoxia) from the EPA National Coastal Condition Report III 2008. Dips in DO are seasonally associated with summer conditions in the shallow estuary. Dissolved oxygen may be expected to decrease with changes in climate trends associated with warming water and air temperatures and other changes in ocean conditions (ie upwelling, wind stress). Also, a 2006 SWMP synthesis report demonstrated that Dissolved oxygen levels in the South Slough estuary are related to ocean conditions.
The Sengstacken monitoring station (omitted here) shows a pattern similar to Winchester; however, from 2007-2010 DO has declined due to site specific problems (isolation of the deployment tube from the main channel especially during low tides); therefore, data is not presented here to prevent skewing for one station.

Figure 3. Average daily dissolved oxygen levels in the South Slough estuary compared with DEQ and EPA standards.

Figure 4. Average daily pH levels in the South Slough estuary compared with DEQ standards.

pH

Long-term water quality data for estuary pH is showing an increase at all stations (Figure 4); the slopes of the trendlines are small (near 0) and the regression values range from 0.05-0.20, indicating a weak positive relationship over the time period. Additional analyses (pH frequency distributions by location along the estuarine gradient and seasonal median pH values) of pH datasets demonstrate this rising trend even further (Figures 5 and 6). This increasing pH trend is unexpected and interesting because it is in the opposite direction of what is happening in the ocean (decrease in pH; ocean acidification). Overall, it is unknown how pH in estuaries may be affected by climate change patterns. One hypothesis as to why estuary pH may be increasing in South Slough is related to Net Ecosystem Metabolism (production and respiration rates). If the South Slough system is experiencing overall higher production than respiration rates ie due to increases in phytoplankton, benthic algae, eelgrass, or salt marsh vegetation over time, then pH would be rising. The black dashed lines indicate Oregon DEQ water quality standards for estuarine and freshwater pH for the South Coast basin. Most of the estuarine pH measurements for South Slough are within this range (6.5 – 8.5) with occasional episodic dips and spikes outside of this range. Freshwaters are naturally more acidic than marine waters therefore the trends observed are consistent with the estuarine salinity gradient.

Figure 5. pH frequency distributions by location along the estuarine gradient.

Figure 6. Seasonal median pH values

Turbidity

Long-term trends in estuarine turbidity show a slight increase in overall turbidity values at all stations (Figure 7). Further analyses to separate out natural turbidity levels due to storm/precipitation events and/or spikes due to potential increases in drift algae has not formally been made. Generally, higher turbidity events are common in the estuary especially during the winter season. Summer peaks may be associated with increases in drift algae and biofouling. If there are increases in storm activities and increase in winter precipitation events, turbidity may be expected to follow ie increased episodic/peaks in values associated with sediment increases and rainfall increases especially during the winter season.
Estuary turbidity criteria are still being developed. Currently, for streams the Oregon DEQ criteria for Turbidity (Nephelometric Turbidity Units, NTU) is: No more than a ten percent cumulative increase in natural stream turbidities may be allowed, as measured relative to a control point immediately upstream of the turbidity causing activity.

Figure 7. Average monthly turbidity levels in the South Slough estuary.

Nutrients: Orthophosphate

The lowest Orthophosphate (PO4) concentrations are observed at upstream WI site, while more marine sites (BH, CH, VA) show higher PO4 concentrations (Figure 8). Orthophosphate shows a weak seasonal pattern with generally higher concentrations during the dry season than the rainy season. Maximum concentrations of orthophosphate at all sites are within a medium eutrophic condition effects category range of 0.01 to

Figure 8. Average monthly orthophosphate levels in the South Slough estuary compared with National Estuarine Eutrophication Assessment standards.

Nutrients: Dissolved Inorganic Nitrogen

Dissolved Inorganic Nitrogen concentrations were highest at the Winchester riverine site than at the more marine/polyhaline sites (BH, CH, VA) and there is a strong seasonal pattern of nitrogen concentrations with higher concentrations occurring during the winter than the summer (Figure 9). Linear regressions of DIN with salinity indicate freshwater nitrogen source are dominant in winter while marine nitrogen sources are dominant in the summer. Maximum concentrations of Dissolved Inorganic Nitrogen at all sites are within a medium/fair eutrophic condition effects category range of 0.1 to

Figure 9. Average monthly dissolved inorganic nitrogen levels in the South Slough estuary compared with National Estuarine Eutrophication Assessment standards.

Bacteria: E. coli compared with tide level

Bacteria monitoring (total coliforms & e. coli) at the SWMP water quality stations has been conducted since 2004 (Figures 10 and 11). This recent one year (2010) snapshot gives an idea of the high variability between sites; the variability is primarily due to the estuarine salinity gradient (bacteria levels are higher at lower salinities). Average E. coli levels are higher at low tides than high tides and during the wet season than the dry season for all sites except Winchester; Winchester low tide bacteria levels are higher during the dry season than the wet season. E. coli bacteria counts fall well below the DEQ standards of 406 organisms/100 mls for most sites except one sampling event at Winchester on 11/18/2010 where E. coli counts at low tide were 421 MPN and at high tide were 2439 MPN per 100 mls.

Figure 10. Monthly E. coli bacteria levels during high tide compared with OR DEQ standard for recreational freshwater and estuarine waters other than shellfish growing areas.

Figure 11. Monthly E. coli bacteria levels during low tide compared with OR DEQ standard for recreational freshwater and estuarine waters other than shellfish growing areas.

Bacteria: Total coliforms compared with tide level

Average total coliforms levels are higher at low tides than high tides and during the wet season than the dry season for the marine to mid-estuary sites (BH, CH, & VA stations) (Figures 12 and 13). At WI, the salinity range is much lower than the BH/CH/VA sites and therefore, dry season counts are higher than wet season for this site; Winchester is also the furthest upstream monitoring site and is influenced heavily by watershed sources therefore bacteria counts are higher at this site for all conditions (tide, season). In particular, notice the anomalous spike event (*counts) that occurred during 11/18/2010 sampling where Total coliform levels at WI are 10x levels at WI during all other sampling events for the year and 20-50x levels at the other sites (BH, CH, VA).

Figure 12. Monthly total coliform bacteria levels during high tide.

Figure 13. Monthly total coliform bacteria levels during low tide.