On Saturday, March 21, 2009, the Seashore and PRNSA hosted Dr. Ann Russell, associate professor of geology at University of California at Davis, to discuss the potential effects of climate change on coastal ecosystems such as the recently restored Giacomini Wetlands. As Dr. Russell noted, changes in the carbon dioxide (CO2) content in the atmosphere have occurred throughout geologic time, but what is unprecedented is the rate and extent to which CO2 is increasing: from a geologic standpoint, it is incredibly rapidly and to a magnitude that has not been documented from previous geologic periods.
Dr. Russell recently began a study on the potential hydrologic effects of climate change on Tomales Bay and other northern California coastal systems. In this study, she is evaluating changes in estuarine circulation and water pH relative to conditions present between 1985 and 1996, when Tomales Bay was the subject of an extensive long-term monitoring program, the Land Margin Ecosystems Research (LMER) and Biogeochemical Reactions in Estuaries (BRIE) projects.
Through research efforts such as these, the Seashore and PRNSA hopes to gain a better understanding of how these global changes will affect our local environment and resources and what we can do to better anticipate-and potentially even mitigate-some of these impacts. Below is a synopsis on what is currently known or believed about climate change and its potential impacts on coastal ecosystems.
When planning for project implementation started, few people other than scientists even talked about global warming, climate change, sea level rise, and the impact on wetlands. However, in recent years, the alarming news stories about the potentially rapidly increasing rate of sea level rise due to glacier melting has catapulted this issue into the public eye and increased concern about the resilience of these important and fragile ecosystems. Increases in sea level could reportedly result in loss of more than 22 percent of the world's wetlands (San Francisco Bay Joint Venture 2008). When combined with the continued loss of wetlands globally to reclamation and development, these losses could climb as high as 77 percent (Nicholls et al. 1999, Najjar et al. 2000 in San Francisco Bay Joint Venture 2008).
Climate change can affect coastal wetland systems through a myriad of direct and indirect effects, including changes in temperature, wind, precipitation, freshwater hydrology, sediment supply and transport, sea level rise, and ocean circulation. With climate change study being a relatively young science, the exact magnitude and extent-and even the direction-of these changes on the northern California coast is still a matter of active debate.
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Sea Level Rise
-- Content for this page was composed by Lorraine Parsons, Project Manager, Giacomini Wetland Restoration Project, Point Reyes National Seashore, May 2010
Sea Level Rise
Based on records from the past century, sea level is rising, and sea level rise increases tidal and wave action, which can destabilize wetland and other coastal ecosystems. In 2005, the USGS completed a relative coastal vulnerability study that depicted most of Tomales Bay as having low to moderate vulnerability to sea level rise (Pendleton et al. 2005). NOAA reports that, based on review of historic (1854-1999) water level gauge data, sea level has risen at a rate of 0.00328 to 0.0079 feet/year over the last century and that sea levels have risen 0.007 feet/year in San Francisco since 1906 (NOAA 2001 in KHE 2006). Based on 25 years of Point Reyes water level records, NOAA estimated a current local sea level rise rate in this region of 0.0082 feet/year or, if there were no change in rate, 7.2 inches or 0.6 feet by 2100 (NOAA 2001 in KHE 2006). Based on recent satellite altimetry studies, Cazenave and Narem (2004) report a "very accurate" sea level rise rate of 0.0092 ± 0.0013 feet/year for the 1993-2003 decade, which, if there were no change in rate, would translate to 0.82 feet by 2100. This rate is notably higher than what NOAA's rate of change based on measured changes in tide gauges over the preceding half century (KHE 2006). However, both of these rates are similar to sea level rise rates reported for the California coast as a whole (8 inches/century), which also closely approximated historical global sea level rise rates.
With the acceleration of climate change impacts, current sea level rise rates are likely to increase, with the exact magnitude of that increase a matter of considerable debate. Earlier this decade, the Intergovernmental Panel on Climate Change (IPCC) developed estimates of sea level rise based on a number of emissions scenarios, with perhaps the most widely accepted rate of rise-at least previously-being 0.5 m (1.65 feet) by 2100. Figure 1 (127 KB PDF) and the map to the right shows what the newly restored Giacomini Wetlands might look were sea level to rise 0.5 m: in this instance, sea level rise would not necessarily convert intertidal vegetated wetland to open water, but could cause a shift "downwards" elevationally in marsh community such that "low" marsh would become more prevalent than "mid" and "high" marsh, and the amount of upland area would decrease. In this case, the potential for upland wetland migration seems eminently feasible, given the relatively large extent of lands not subject to tidal action in the Project Area (~190 acres, with some of this being wet meadow or other seasonal wetland). In general, more than 50 percent of the Marin County coastline has viable area for upland wetland expansion (The Pacific Institute 2009). However, contrasted with that is the situation in many portions of San Francisco Bay where lower elevation wetland directly abuts developed areas, with no undeveloped upland available for wetland migration.
More recently, researchers from University of Arizona, the National Center of Atmospheric Research, and other institutions suggest that accelerated melting of the Arctic and Antarctic ice caps and Greenland glaciers could raise sea level by as much as 3 feet by the end of this century and 13 to 20 feet in coming centuries (Overpeck et al. 2006; Velicogna and Wahr 2006). Cayan et al. (2008) projected that sea level rise rates in California could increase to between 11 and 72 cm (0.4 and 2.4 feet) by 2079-2099. A recent study suggests that even some of these more alarming numbers could be underestimates due to the fact these models-including the IPCC ones-did not incorporate key forces such as gravity and changes in the Earth rotation, leading to potentially another 4 to 5 feet in sea level rise if the West Antarctic Ice Sheet collapses (University of Toronto 2009). In 2009, the Pacific Institute published a report that warned that sea level rise rates in California could climb as high as 1.4 m (4.6 feet) by 2100 (The Pacific Institute 2009). Should sea level rise that high, the report estimated that as much as 41 sq. miles or 26,000 acres of California coastline could be lost due to increases in wave erosion and amplified tides (The Pacific Institute 2009).
Precipitation, Run-Off, and Sedimentation
By their very nature, wetlands are dynamic interfaces between sea and land. Over geologic time, the position or location of marshes in the estuarine landscape have shifted in response to long-term changes in sea level (i.e., advancing with warming of temperatures and retreating with cooling of temperatures) and trends in sediment transport and deposition from the source watershed. Most of the terrestrially derived sediment supplied to coastal areas comes during storm events in the form of suspended sediment in run-off. Therefore, changes in precipitation and run-off may dictate to some extent how viable coastal systems remain in the face of a greatly accelerated sea level rise rate.
On average, recent projections show little change in total annual precipitation in California or in the Mediterranean pattern of rainfall, with most falling during winter from north Pacific storms (California Climate Change Center 2006). However, one climate model does predict slightly wetter winters (+38 mm/yr), while another predicts slightly drier winters with a 10 to 20 percent decrease in precipitation (-157 mm/yr; Cayan et al. 2006 in Dukes and Shaw 2007; California Climate Change Center 2006).
Sacramento Delta and San Francisco Bay wetlands may be subject to greater change than coastal systems due to decreases in the amount of snowpack in the Sierra Nevada Mountains and associated effects on timing and volume of stream and river flow during the winter and spring and freshwater inflows to downstream systems. Wetter winters and springs with less snowpack translates into higher, more concentrated periods of runoff that could increase erosion within the upper watersheds and downstream transport of sediment (San Francisco Bay Joint Venture 2008). The effects of wetter, warmer winters may be less magnified in coastal wetlands than in those fed by higher mountain ranges, because precipitation from source watersheds does not typically fall as snow, but prolonged wet periods could change the type of downstream vegetation communities or even wetland types (i.e., brackish versus salt marsh), although, along the coast, this increased freshwater influence would be countered by sea level rise. Based on results from recent computer modeling by KHE (2009), sea level rise in the Giacomini Wetlands would be accompanied by dramatic increases in salinity that could also change marsh dynamics by converting brackish and even freshwater marsh communities to salt marsh ones.
The long-term resilience of coastal systems will be dependent to a large degree on continued sediment inputs from upstream portions of the watershed. Without these inputs, marshplains will sink below the steadily creeping rise in sea level and convert to non-vegetated types of wetlands such as intertidal mudflat or subtidal or open water habitat (San Francisco Bay Joint Venture 2008). Systems that are already sediment-starved to some degree such as the San Francisco-Sacramento Delta estuary-which has lost much of its sediment supply to upstream dams-are in greater danger from sea level rise than those in systems with either natural or even unnaturally higher rates of downstream sediment delivery (Orr et al. 2003). Somewhat ironically, the potential for higher amounts of rainfall and associated run-off could help to counter the effects of sea level rise for some coastal ecosystems, particularly those where snowpack currently mediates the intensity and timing of run-off.
Within Tomales Bay, sediment delivery rates are believed to have dropped from peak levels between the late 1800s and early 1900s, but still to be higher than what occurred prior to agricultural development and lumber harvesting. The highest rates of sedimentation at the southern end of Tomales Bay occurred between 1861 and 1908 (PWA et al. 1993), but, apparently for other portions of the Bay, the largest sediment influx to the bay seems to have been in the decades between about 1930 and 1960 (Rooney and Smith 1999). Since 1957, sedimentation rates have dropped, slowing the pace of Bay infilling (Rooney and Smith 1999, PWA et al. 1993). Between 1861 and 1931, sedimentation accumulation rates within Tomales Bay averaged 94 tons per square kilometer per year, increasing to 357 tons/k2/yr between 1931 and 1957 and decreasing to 101 tons/k2/yr between 1957 and 1994 (Rooney and Smith 1999). Sedimentation resulting from erosion induced by agricultural development of the watershed is likely to have been highest first at the southern end or mouth of the Bay, with the rapidly accreting delta and construction of the Giacomini Ranch levees eventually shifting the primary area still available for sediment deposition downstream and more into the Bay itself. While watershed sediment contribution has decreased in the last 50 years, Tomales Bay continues to become shallower through sediment inputs. The present sedimentation rate in the bay, based on both bathymetric changes since 1957 and sediment yield measurements, is estimated at about 0.04 to 0.08 in/yr (Smith and Hollibaugh 1998).
Another factor that may increase resilience of coastal marshes to sea level rise, particularly marshes in sediment-starved systems, is increase in surface elevation associated with organic, rather than mineral, matter. A recently published study by Langley et al. (2009) found that the elevation of salt marshes increased in areas simply through an increase in root biomass, even when where the contribution of mineral sediment deposition was negligible. Not only is surface elevation increased through increases in root biomass, but also through accumulation of undecomposed plant matter or peat, and systems exposed to frequent sedimentation or flooding events are likely to develop more layers or strata of peat that contribute to increases in marsh surface elevation. In one study of 55 wetlands in the Gulf of Mexico and three (3) wetlands in Rhode Island, Turner and colleagues found that organic matter accumulation represented the "dominant influence" on vertical accretion in salt marshes (Turner et al. 2000), which supported earlier findings published by Callaway and colleagues demonstrated a strong statistical relationship between organic matter accumulation and vertical accretion rates in 5 Gulf of Mexico wetlands (Callaway et al. 1997).
Coastal Winds and Temperature
Climate change may also be associated with a change in wind patterns and strength. A recent study showed that land temperatures are increasing at a faster rate than ocean temperatures, and this steepening of the thermal gradient from inland to sea is already resulting in increased winds (Snyder 2008). The University of California, Santa Cruz team ran several regional climate change models based on modern climate (1968 to 2000) and future climate (2038 to 2070) using input from the IPCC AR4 for "high-growth" emissions scenarios. Results showed an increase in wind speed of up to 2 meters/second, which is a large change relative to the current average wind speed of 5 meters/second (Snyder 2008).
Ironically, while climate change is predicted in general to increase ambient air temperature by 1.7 degrees Centigrade (3.0 degrees Fahrenheit) to as much as 5.8 degrees Centigrade (10.4 degrees Fahrenheit) by the end of this century (Cayan et al. 2006 in Dukes and Shaw 2007, Cayan et al. 2008), the increase in temperature gradient along the coast may actually decrease temperatures along the coast (Snyder 2008). An increase in winds can amplify tidal range or the upward extent of high tides and induce wave-associated erosion of shorelines. Winds are generally milder in the southern portion of Tomales Bay relative to the coast, but these conditions may change with regional shifts in wind patterns and strength. In 2008, unusually strong winds in southern Tomales Bay may have led to amplification of tides within certain portions of Lagunitas Creek by as much as 0.5 to 0.7 feet.
Cooler temperatures along the coast would seemingly favor continuation of the fog "belt" that typically cloaks the coast during the summer and days when temperatures soar in inland areas. Fog in northern California is strongly related to the phase of the Pacific Decadal Oscillation (PDO), as well as interannual variability in coastal sea surface temperature and sea level pressure over the interior western United States (J. Johnstone, UC Berkeley, unpub. data). The frequency of fog closely approximates the temperature differential between coastal and inland weather stations (J. Johnstone, UC Berkeley, unpub. data). Interestingly, while predicted changes discussed above would suggest a potential increase in fog, monitoring shows that the frequency of fog has varied substantially over the last 100 years, but, in general, fog frequency has declined 33 percent from 1900 (Johnstone and Dawson 2009). Fog frequency dropped from 44 percent between 1951 and 1975 to 27 percent in 1997, but rebounded somewhat in subsequent years to 42 percent (Johnstone and Dawson 2009). Over this time period, the temperature differential between inland and coastal areas has been dropping at a rate as high as 3.8 degrees Centigrade per century, which would result in generation of less fog (Johnstone and Dawson 2009).
This historical observation, of course, is somewhat at odds with the near-future predictions of Snyder and colleagues, who, as noted earlier, have postulated that the gradient between inland and coastal temperatures will increase as temperatures along the coast become cooler (Snyder et al. 2003, Snyder 2008). As a result, fog development and onshore flow during summer months could intensify (Snyder et al. 2003). Given the seeming contradiction between recent historical trends and future modeling predictions, it would potentially seem that the cooling of temperatures associated with increases in upwelling could cancel out the increase in warming associated with the greenhouse effect, and the historical trend documented by Johnstone and Dawson (2009) may reverse (M. Snyder, T. O'Brien, University of California Santa Cruz, pers. comm.).
Changes in the coastal wind patterns would not only affect temporal and spatial patterns of fog, but would potentially affect coastal upwelling. Coastal upwelling or the turnover of deep ocean waters during the summer along the Tomales Bay and Marin coast drives the amazing productivity of nearshore waters that creates a richness and diversity in aquatic life documented in few places in the world.
Stronger coastal winds and temperature differentials between warmer inland and cooler coastal areas may increase the strength and intensity of coastal upwelling (Schwing and Mendelssohn 1997, Mendelssohn and Schwing 2002, Snyder et al. 2003, Barth et al. 2007). In contrast, some have predicted that stronger thermal stratification and a deepening of the thermocline could prevent cool, nutrient-rich waters from being upwelled (Roemmich and McGowan 1995), but paleoclimatic data suggests that upwelling in the California current system, as well as reduced avection in terms of ocean circulation, is positively correlated with temperature over millennial timescales (Pisias et al. 2001). While seemingly an increase in upwelling might seem on the surface to be potentially advantageous, increased upwelling could result in an increase in hypoxic events in nearshore and estuarine waters due to an over-abundance of nutrients brought up to the surface by turnover of deep, nutrient-rich ocean waters (J. Dorman, UC Berkeley, unpub. data). Researchers believe that too much upwelling may have caused the massive "dead zone" that has begun to appear with alarming regularity off the Oregon coast: Stronger, more persistent winds lead to more intense upwelling that stimulates excessive growth of phytoplankton, which sink ultimately to the bottom and decompose, sucking oxygen out of the bottom waters (Snyder 2008). These conditions may become more prevalent with climate change in the future, extending this dead zone phenomenon into California coastal waters (Snyder 2008).
In addition to hypoxia, changes in upwelling may have even more direct effects on aquatic communities dependent on this phenomenon. Both phytoplankton and zooplankton densities appear to peak in abundance when wind speeds along the coast are below maximum levels-somewhere around 7 to 12 mph (Botsford et al. 2003). Higher average wind speeds could depress nearshore ocean and estuarine productivity.
In addition to potential increases in intensity, climate change may also result in delayed upwelling (Schwing and Mendelssohn 1997, Mendelssohn and Schwing 2002, Snyder et al. 2003, Barth et al. 2007). Upwelling typically starts in March or April, peaks in July, and ends abruptly in October. With climate change, early season upwelling could be delayed, and the strength or intensity of late season upwelling could be increased (Schwing and Mendelssohn 1997, Mendelssohn and Schwing 2002, Snyder et al. 2003, Barth et al. 2007). These changes in temporal patterns of upwelling could result in prey populations of phytoplankton and other microscopic organisms that are highly sensitive to fluctuations in nutrients blooming later when predator populations are low, leading to so-called trophic mismatch (Durant et al. 2007). In 2005, upwelling was weak and delayed several months due to unusually warm waters and other factors: that same year, reproductive success of the planktivorous coastal bird species, Cassin's auklet (Ptychoramphus aleuticus) plummeted, presumably in response to low prey availability (Schwing et al. 2006). Not all of the effects from phenomena such as this are immediate. In 2007, escapement rates of chinook salmon (Oncorhynchus tshawytscha) crashed: an extensive evalution of potential causes by fisheries eventually led them to hypothesize that the declines occurred when the 2005 smolts entered nearshore waters-only to find low food supplies. Should upwelling be delayed in future years, seasonal declines in species such as krill (Euphausia pacifica) could have devastating effects on species such as chinook salmon and Cassin's auklet.
Tomales Bay is one of the beneficiaries of the incredible productivity in nearshore waters during upwelling events. The most intensive upwelling occurs in Tomales Bay during the summer, in response to strong, often persistent northwesterly winds (Smith and Hollibaugh 1997). Upwelling elevates the concentration of particulate organic matter in the coastal waters, which is then delivered to the bay by tides and particle settling (Smith and Hollibaugh 1997). Direct inorganic nutrient delivery from coastal upwelling in the Pacific Ocean is not of major importance to Tomales Bay, but may be important indirectly by affecting nutrient dynamics or cycling within the bay (Smith and Hollibaugh 1997). While upwelling may not drive nutrient dynamics in the southern portion of the bay as strongly as it does the outer portions of the Bay, organisms from the outer Bay do move up into the uppermost reaches of the estuary, and decreased productivity in the Bay itself could lead to decreased productivity in the Giacomini Wetlands area. In addition, an increase in carbon or inorganic nutrient delivery to Tomales Bay could result in waters, which were characterized by the LMER/BRIE researchers as being relatively low in nutrients and non-eutrophic (Cole 1989; Chambers 2000; Lewis et al. 2001), becoming more prone to eutrophication and hypoxic events, all of which could affect upstream waters both directly during incoming tides and indirectly.
Acidification of Coastal Waters
Other changes that may affect the Tomales Bay estuary include the acidification of estuarine waters from deposition of carbon dioxide from the atmosphere, as well as over-fertilization of native habitats due to increased deposition of nitrogen from the atmosphere. Dissolved inorganic carbon (DIC) exists in seawater in three major forms: bicarbonate ion (HCO3-), carbonate ion (CO32-), and aqueous carbon dioxide (CO2), which here also includes carbonic acid (H2CO3). At a pH of 8.2, approximately 88 percent of the carbon is in the form of HCO3-; 11 percent is in the form of CO32-; and only 0.5 percent of the carbon is in the form of dissolved CO2 (Fabry et al. 2008). When CO2 dissolves in seawater, H2CO3 is formed: Most of the H2CO3 quickly dissociates into a hydrogen ion (H+) and HCO3- (Fabry et al. 2008). The atmospheric CO2 value currently is ~100 ppm greater than the pre-industrial value (280 ppm), and roughly half (48%) of the CO2 released by human activities between 1800 and 1994 is now stored in the ocean (Sabine et al. 2004). As a result, the average surface ocean pH has dropped by 0.1 unit, which is about a 30 percent increase in H+ (Fabry et al. 2008). Under the IPCC emission scenarios (Houghton et al. 2001), average surface ocean pH could decrease by 0.3 to 0.5 pH units from the pre-industrial values by the end of this century and by 0.3 to 1.4 units over the next 300 years, depending on the emissions scenario used (Caldeira and Wickett 2005).
This decrease in pH is important because many of the organisms in the ocean rely on calcium carbonate processes to form shells or body parts: when alkalinity drops below a certain level, shells are more likely to dissolve than to form. This phenomenon can be observed visually by pitting in shells or other morphological indicators (A. Russell, UC Davis, pers. comm.). The extent to which such organisms are affected depends largely upon the saturation state for either aragonite or calcite, two types of CaCO3 commonly secreted by marine organisms: In regions where the saturation state of aragonite or calcite is >1.0, formation of shells and skeletons is favored (Fabry et al. 2008). For values <1.0, seawater is corrosive to CaCO3, and, in the absence of protective mechanisms (e.g. Corliss and Honjo 1981; Isaji 1995 in Fabry et al. 2008), dissolution will begin. While many organisms have adapted to thermal fluctuations in the last few million years, the expected changes in pH are higher than any other pH changes inferred from the fossil record over the past 200 to 300 million years (Caldiera and Wickett 2005, Feely et al. 2004).
Not only can this issue affect calcareous organisms, but it can have a cascade effect on the food web. Pteropods are an important component of the diet of pink and some other types of Pacific Northwest salmon. Pteropods form shells made of aragonite, which is approximately 50% more soluble in seawater than calcite (Mucci 1983 in Fabry et al. 2008). Owing to their highly soluble aragonitic shells, pteropods may be particularly sensitive to ocean acidification (Fabry et al. 2008). In a three-year study designed to examine the interannual variability of the feeding habits of juvenile pink salmon, Armstrong et al. (2005) found that pteropods accounted for ≥ 60 percent by weight of the juvenile salmon diet in two of three years. In a model study linking oceanic foodwebs to production and growth rates of pink salmon, Aydin et al. (2005) found that salmon expended less energy in the early spring when foraging on zooplankton than squid, although squid was more important later in the year. Other preliminary model results suggest that a 10 percent decrease in pteropod production could lead to a 20 percent drop in mature pink salmon body weight (Aydin, pers. comm. in Fabry et al. 2008), and body weight for salmon is positively correlated with survival at sea.
As part of her study, Russell has been re-evaluating the long-term dataset available for Tomales Bay from the LMER/BRIE program, because it includes alkalinity data. Between 1985 and 1996, pH of Tomales Bay waters varied seasonally throughout the 11-year dataset, with decreases in pH during the wet season (A. Russell, UC Davis, pers. comm.). A drop in pH was documented during the latter years of the program, but the cause of this shift is unknown and could potentially be related to easing of the below-average or drought conditions that occurred throughout most of the study period. Changes in pH and saturation levels of aragonite and calcite could prove important in an estuary that supports an active mariculture industry.
At least one study predicted that mussel and oyster calcification rates could decline by as much as 25 percent and 10 percent, respectively, using the now seemingly optimistic IPCC scenarios (Gazeau et al. 2007). Another laboratory study by Bodega Marine Laboratory looked at calcification of larvae and juveniles produced by adult oysters from Tomales Bay (Gaylord 2010). Carbon dioxide concentrations in laboratory seawater were controlled to match present-day conditions in the oceans, 380 parts per million (ppm), as well as two carbon dioxide scenarios projected to occur by the year 2100 (540 and 970 ppm). The bottom-dwelling juveniles in the 970 ppm treatment grew 41 percent less than juveniles under control conditions, and the consequences persisted to a large degree, even after juveniles were returned returned to present-day CO2 conditions, with juveniles still 28 percent smaller than those grown in control conditions.
The vulnerabily of estuaries such as Tomales to acidification impacts is still unclear. Some feel that estuarine and coastal ecosystems may be especially vulnerable to changes in water chemistry because their relative shallowness, reduced salinity, and lower alkalinity makes them inherently less buffered to changes in pH than in the open ocean. However, others note that estuaries-and the organisms that live there-may be less susceptible to the effects of acidification than those living in oceans, because pH is generally more variable in estuaries.
Changes in Estuarine Salinity with Sea Level Rise
What may have a greater effect on estuaries than changes in pH are changes in salinity with sea level rise and potential changes in freshwater inflow. As noted earlier, changes in salinity can change not only the type of wetland (salt vs. brackish vs. fresh), but the assemblage of species. Where this becomes particularly important is for those rare and perhaps even listed plant and wildlife species that rely on a specific water regime for persistence.
For example, Giacomini Wetlands supports several listed species that rely on specific water regimes for persistence. The federally threatened California red-legged frog (Rana aurora draytonii) is not an estuarine species, however, it often occurs in freshwater pockets along the edges of estuaries. Some studies have shown that eggs of red-legged frog are unlikely to persist in salinities higher than 4.5 ppt, and that larvae and adults are typically not found in areas where salinities exceed 7 ppt (Jennings and Hayes 1989). (As a point of comparison, ocean salinity in northern California is estimated at 34 ppt.) Another species, tidewater goby (Eucyclogobius newberryi), a federally endangered fish resident in estuaries, is considered a brackish species that prefers salinities around 10 ppt, but can persist in waters where salinities reach 25 ppt or higher (Swift 2003). While not observed in recent times, the federally endangered California freshwater shrimp (Syncaris pacifica) does occur in the Lagunitas Creek watershed, and, as its name implies, is reliant on having freshwater or very low salinity conditions.
Some updated hydrodynamic modeling recently conducted by KHE for the Giacomini Wetlands suggested that sea level rise rates between 1 and 3 feet would have major implications for salinity regimes within the newly restored wetlands-and, by association, for the species that live or visit there. Average salinity increase-represented by the 50% or median salinity concentration)-increased 5.8 psu (102%) and 3.2 psu (409%) in upstream portions of the Giacomini Wetlands at the Lagunitas Creek-Bear Valley Creek confluence and Green Bridge locations, respectively (KHE 2009). Based on modeling, not only would habitats migrate upland, but changes in salinity structure with sea level rise could potentilly compress the amount of habitat available for organisms such as red-legged frog and tidewater goby. Figure 2 (322 KB PDF) shows a snapshot of salinity conditions along a longitudinal gradient in the Giacomini Wetlands with 1 foot of sea level rise. While salinities would obviously continue to vary seasonally, this graphic suggests that available habitat for red-legged frog and tidewater goby would migrate relative to existing available habitat and potentially shrink in size. However, this represents a conservative estimate of available habitat over a larger geographic scale and does not take into account seasonal variation in salinity or smaller "pockets" of available habitat that may persist that cannot be captured at this scale.
One other atmospheric component to climate change involves increased deposition of nitrogen. Throughout California, high levels of ammonia and nitrogen oxides are emitted to the atmosphere by nitrogen fertilizer use and feedlots in agricultural areas and internal combustion engines in urban areas (Dukes and Shaw 2007). Nitrous oxide production can be higher in warmer climates (Carter et al. 1979, Kotchenruther et al. 2001), so warming associated with increases in atmospheric CO2 could also affect atmospheric nitrogen concentrations. Although extremely patchy, nitrogen deposition rates in parts of California, including its streams, are among the highest in the United States (Fenn et al. 2003 2007), with rates of up to 45 kg/hectare/yr in southern California; 16 kg/hectare/yr in northern California, and up to 90 kg/hectare/yr in areas of extensive fog exposure (Bytnerowicz and Fenn 1996, Padgett et al. 1999, Fenn et al. 2003, Blanchard and Tonnesen 1993 in Dukes and Shaw 2007). In southern California, most of the deposition occurs in summer as dry deposition, while in northern California, most occurs as wet deposition (Bytnerowicz and Fenn 1996; Blanchard and Tonnesen 1993 in Dukes and Shaw 2007). Under one future scenario (IPCC A2 scenario), global nitrogen deposition was estimated to increase by as much as 250 percent, mostly because of increases in nitrogen deposition (Lamarque et al. 2005).
Salt marshes are typically considered to be nitrogen limited systems (Day et al. 1989, Mitsch and Gosselink 2000). This nitrogen limitation drives community assemblage and species interactions. In both marine and estuarine systems where nitrogen supplies are elevated, ecosystem structure is often fundamentally altered, leading to an explosion in non-native species that flourish under high-nitrogen conditions and are able to outcompete native species. In grasslands, nitrogen supplementation has been associated with increases in plant biomass (usually exotics); decreases in species diversity; and decreases in diversity and abundance of native grasses and herbs and increases in dominance by one or two species of non-native grasses (Dukes et al. 2005, Huenneke et al. 1990).
Carbon Sequestration and Interactions of Carbon with Marsh Vegetation Communities
There can be interaction between coastal wetlands and atmospheric carbon, as well as nitrogen. However, in the case of carbon, marshes may perform a valuable function in mediating effects of climate change, just as do forests. In fact, tidal marshes may be more efficient than forests at carbon sequestration, as it is called, on per unit area (Trulio et al. 2007). Tidal marshes, in particular, are extremely productive habitats that capture significant amounts of carbon from the atmosphere (Trulio et al. 2007). Tidal marshes can remove carbon as carbon dioxide from the atmosphere by producing up to 8,000 metric tons of plant material per year (Mitsch and Gosselink 2000 in Trulio et al. 2007). Upon decomposition, much of this material is stored long-term in soils, unlike forests, where carbon is stored in trees. Some rates estimated for south San Francisco Bay wetlands were as high as 54 g to 385 g of carbon per square meter per year (Chmura et al. 2003). In a review on North American wetlands, Brigham and colleagues noted that, "estuarine wetlands sequester carbon at a rate about 10-fold higher on an area basis than any other wetland ecosystem due to high sedimentation rates, high soil carbon content, and constant burial due to sea level rise" (Bridgham et al. 2006). In addition, unlike freshwater wetlands, tidal marshes release only negligible amounts of methane, which is a powerful greenhouse gas (Choi et al. 2004). The carbon sequestration rates of undisturbed wetlands were lower (15 percent for mangrove and 55 percent for saltmarsh) than disturbed wetlands, although the carbon store was higher (65 percent for mangrove and 60 percent for saltmarsh), suggesting that "wetland rehabilitation has positive benefits for regulation of atmospheric carbon concentrations, in addition to more broadly accepted ecosystem services" (Howe et al. 2009).
Ultimately, carbon sequestration could change the character of coastal marshes by favoring species with enhanced carbon uptake rates over species with lower rates. Essentially, increasing CO2 is like adding fertilizer. For some species, increased CO2 uptake results in higher shoot density and biomass and higher root biomass (Langley et al. 2009). In higher CO2 environments, so-called C3 plants appear to have an edge over C4 plants. C3 plants, which are so named because the first step of photosynthesis involves a three carbon-molecule, is the original and more common style of photosynthesis, which evolved more than 600 million years ago when atmospheric CO2 was extremely high (Baskin 1994). C3 plants include key crops such as wheat, rice, and beans and virtually all trees. At today's CO2 concentrations, this carbon-capturing mechanism is somewhat inefficient, with plants losing as much as half the carbon they garner, but under higher levels, this loss is eliminated (Baskin 1994). C4 plants, on the other hand, have a more efficient form of photosynthesis that creates a four-carbon molecule in the first stage and prevents carbon loss (Baskin 1994). This type of photosynthesis evolved later, probably at the end of the Cretaceous Period (some 65 million years ago), when CO2 began to drop (Baskin 1994). These plants do not seemingly respond to increases in atmospheric CO2. Though fewer in number, C4 plants also include important crops-corn, sorghum, sugarcane, pineapples-as well as prairie and savanna grasses and many shrubs (Baskin 1994).
Should C3 species have an evolutionary advantage over C4 plants in richer carbon environments, the communities within coastal marshes could shift. One early experiment in Maryland's Chesapeake Bay tracked C3 sedges and C4 cordgrass growing in a coastal salt marsh in open-top chambers that exposed them to double current levels of CO2 for at least five years (Baskin 1994). With nutrients in marshes relatively plentiful, the C3 sedges responded with increased photosynthesis and growth, while the C4 cordgrass showed little response to the increased gas and lost ground to the sedges (Baskin 1994). Many brackish marshes and mangrove swamps are dominated by C3 plants (Langley at al. 2009). C3 plants, at least in the Chesapeake Bay region, also tend to be more flood tolerant than C4 plants, which are more salt tolerant (Langley et al. 2009). While some have theorized that this carbon enrichment of C3 plants is not long-term and that there is an acclimation or leveling off in plant growth after a few years, others dispute this theory, pointing to sustained increases in biomass over longer periods of time with no leveling off (Johnson et al. 1993). Certainly, even if carbon uptake and photosynthesis, biomass increases can be inhibited by other factors, such as water, sunlight, nitrogen, and phosphorous, such that the factor limiting growth-and there always is one-simply shifts.
Biomass increases are not just limited to aboveground portions of plants, but can extend into the soil, and, as discussed earlier, this factor may help to alleviate some of the impacts from sea level rise in marshes. While there was a slight loss in elevation (-0.9 mm/yr) in the ambient CO2 plot, there was a net gain (3.0 mm/yr) in the elevated CO2 treatment (Langley et al. 2009). Increases in root zone thickness appeared to account for most of this elevation gain, and, not surprisingly, vertical changes in elevation were positively correlated with subsurface shoot volume of the C3, but not C4, species (Langley et al. 2009). Interestingly, higher levels of nitrogen, as might occur in increased nitrogen deposition scenarios as described above, appeared to potentially affect elevation negatively (Langley et al. 2009). Elevation of tidal marsh soils in the vicinity of plants with higher CO2 uptake rates may enable these species or communities to better persist in face of sea level rise than those with lower CO2 uptake rates.
The mounting volume of information now being published on the effects of climate change suggest that our coastal ecosystems could change greatly in the coming century and that changes for coastal wetlands along the coast may be vastly different from those in San Francisco Bay and other systems fed by snowpack melt during the spring and early summer. By understanding the potential trajectory of these complex and diverse physical, hydrologic, and ecological changes, resource managers can work to increase the resiliency of these valuable ecosystems and perhaps even help to mediate some of the impacts of our changing climate through continued restoration.
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