Southern California Salt Marsh Research Articles

Characterizing patterns of plant distribution in a southern California salt marsh using remotely sensed topographic and hyperspectral data and local tidal fluctuations

We used LiDAR topographic data, AVIRIS hyperspectral data, and locally measured tidal fluctuations to characterize patterns of plant distribution within a southern California salt marsh (Carpinteria Salt Marsh (CSM)). LiDAR data required ground truthing and correction before they were suitable for use. Twenty to forty percent of the uncertainty associated with LiDAR was due to variance in the elevation of the target surface, the balance was attributed to error inherent in the LiDAR system. The incidence of LiDAR penetration of plant canopy cover (i.e., registration of ground elevation) was only three percent. The depth of LiDAR penetration into the plant canopy varied according to plant species composition; plant species-specific corrections significantly improved LiDAR accuracy (58% reduction in overall uncertainty) and with the use of ground-based surveys, reduced overall RMSE to an average of 6.3 cm in vegetated areas. A supervised classification of AVIRIS data was used to generate a vegetation map with six classification types; overall classification accuracy averaged 59% with a kappa coefficient of 0.40. The vegetation classification map was overlaid with a LiDAR-based digital elevation model (DEM) to compute elevation distributions and frequencies of tidal inundation. The average elevations of the dominant plant classifications found in CSM (e.g., Salicornia virginica, Jaumea carnosa, and salt-grass mix, a mixture of multiple marsh plant species) occurred within a 17 cm range, a vertical change that resulted in a 7% difference in the period of tidal inundation.

Invasion of tamarisk (Tamarix spp.) in a southern California salt marsh

Exotic plants have been demonstrated to be one of the greatest threats to wetlands, as they are capable of altering ecosystem-wide physical and biological properties. One of the most problematic invaders in the western United States has been salt cedar, Tamarix spp., and the impacts of this species in riparian and desert ecosystems have been well-documented. Here we document large populations of tamarisk in the intertidal salt marshes of Tijuana River National Estuarine Research Reserve, a habitat not often considered vulnerable to invasion by tamarisk. Initial research demonstrates that there are multiple species and hybrids of Tamarix invading the estuary and that the potential impact of tamarisk within this salt marsh is significant. This highlights the need for managers and scientists to be aware of the problems associated with tamarisk invasion of coastal marine habitats and to take early and aggressive action to combat any incipient invasion.

Accelerating the Restoration of Vegetation in a Southern California Salt Marsh

Re-establishing plant cover is essential for restoring ecosystem functions, but revegetation can be difficult in severe sites, such as salt marshes that experience hypersalinity and sedimentation. We tested three treatments (adding tidal creeks, planting seedlings in tight clusters, and rototilling kelp compost into the soil) in a site that was excavated to reinstate tidal flows and restore salt marsh. The magnitude of responses was the reverse of expectations, with tidal creeks having the least effect and kelp compost the most. On the marsh plain, kelp compost significantly increased soil organic matter (by 17% at 0–5 cm; p = 0.026 and 11.5% at 5–20 cm; p = 0.083), total Kjeldahl nitrogen (45% at 5–8 cm; p 50% increase in a growth index at 20 months after planting; p < 0.0001). In Spartina foliosa plots, kelp compost did not affect soil organic matter, but plants were taller (by ~11 cm; p = 0.003) and denser (47% more stems; p = 0.003). Planting seedlings 10-cm apart in tight clusters on the marsh plain increased survivorship by 18% (compared to 90-cm apart in loose clusters; p = 0.053), but not growth. Tidal creek networks increased survivorship of Batis maritima and Jaumea carnosa by ≥20% (p = 0.060 and 0.077, respectively). Kelp compost had a strong, positive influence on vegetation establishment by ameliorating some of the abiotic stress.

LARVAL SWIMMING OVERPOWERS TURBULENT MIXING AND FACILITATES TRANSMISSION OF A MARINE PARASITE

Planktonic cercariae (parasite larvae) of digenetic flatworms (Himasthla rhigedana) encyst up to 100% of intermediate host populations. Toward explaining such high prevalence, larval behavior and passive-transport processes were evaluated experimentally for their roles in waterborne parasite transmission. Using a new application of laser and digital video imaging technologies, we quantified cercarial movements in still water and in simulated field flows. In still water, downward swimming in response to light, irrespective of intensity or source, and gravity brought larvae to the bottom three-times faster than gravitational sinking alone. A 33% elevation in temperature (18–24°C) caused a 71% increase in swim speed. In flume flows characteristic of southern California salt marshes (u* = 0.2 cm/s, occurring >80% of the time), vertical larval distributions were highly bottom skewed. The mean downward swim speed (0.59 cm/s at 24°C) was three times faster than turbulent fluctuations (w′ = 0.23 cm/s), indicating that cercariae overpowered eddies to reach the bed. When flume flows simulated rare storm events (u* = 0.8 cm/s, w′ = 0.95 cm/s, occurring ≤1% of the time), turbulence overwhelmed behavioral effects and homogeneously mixed larvae throughout the water column. Environmentally triggered downward swimming can thus quickly bring larvae to the bed, promoting contact with benthic intermediate hosts (snails and crabs) under most flow conditions. The efficacy of vertical swimming in typical marsh flows identifies larval behavior, not passive transport, as the principal mechanism responsible for dispersal to host habitat.

A Comparison of Remote Sensing and Ground‐Based Methods for Monitoring Wetland Restoration Success

Abstract Efficient and accurate vegetation sampling techniques are essential for the assessment of wetland restoration success. Remotely acquired data, used extensively in many locations, have not been widely used to monitor restored wetlands. We compared three different vegetation sampling techniques to determine the accuracy associated with each method when used to determine species composition and cover in restored Pacific coast wetlands dominated by Salicornia virginica (perennial pickleweed). Two ground‐based techniques, using quadrat and line intercept sampling, and a remote sensing technique, using low altitude, high resolution, color and color infrared photographs, were applied to estimate cover in three small restoration sites. The remote technique provided an accurate and efficient means of sampling vegetation cover, but individual species could not be identified, precluding estimates of species density and distribution. Aerial photography was determined to be an effective tool for vegetation monitoring of simple (i.e., single‐species) habitat types or when species identities are not important (e.g., when vegetation is developing on a new restoration site). The efficiency associated with these vegetation sampling techniques was dependent on the scale of the assessment, with aerial photography more efficient than ground‐based sampling methods for assessing large areas. However, the inability of aerial photography to identify individual species, especially mixed‐species stands common in southern California salt marshes, limits its usefulness for monitoring restoration success. A combination of aerial photography and ground‐based methods may be the most effective means of monitoring the success of large wetland restoration projects.

Effects of Sewage Sludge on the Growth of Potted Salt Marsh Plants Exposed to Natural Tidal Inundation

Abstract A growth experiment with native plants in pots exposed to natural environmental conditions evaluates the use of sewage sludge as a soil amendment in restoration of a southern California salt marsh. Sludge containing desirable organic matter but also undesirable heavy metal contaminants was mixed with a readily available matrix soil to reduce metal concentrations to levels below legal limits for land applications of sludge. Soil nutrient analysis revealed expected increases in total nitrogen and total phosphorus content with increasing sludge concentration. Soil metals analysis, however, revealed decreases in metal content with increased sludge concentration, a trend evidently caused by higher than expected metal content in the matrix soil. Five artificial soil mixtures ranging from 0 to 70% sludge were accompanied by natural wetland soil controls. Pots containing these soils were placed into a natural salt marsh. The pots were then planted with two native salt marsh plant species, Salicornia virginica and Frankenia grandifolia. Aboveground biomass was harvested after 12 months. Plant growth displayed no obvious change with increasing sludge concentration. Over the concentration ranges used, increased nutrient content did not stimulate plant growth and increased metal content did not inhibit plant growth. Plants grew better in natural wetland soil than in artificial soil mixtures, a trend probably caused by the substantially finer texture and higher organic content of natural soil. All sludge treatments differed more from the natural soil than from each other, implying that within the ranges examined, soil texture and organic content exerted more influence on plant growth than did metal or nutrient concentration. These results suggest that incorporating this sewage sludge in the soil of the restored salt marsh will neither benefit nor harm the plants that will live there and that greatest plant growth will be achieved by mixing the sludge with a fine‐grained matrix soil.

Halophyte recruitment in a salt marsh restoration site

In restored salt marshes, seedling recruitment can be limited where large areas of soil are exposed and physical conditions are harsh. On a 0.7-ha excavated marsh plain, we studied recruitment as a function of abiotic (elevation) and biotic factors in 2 × 2 m plots planted with 0, 1, 3, or 6 species from the pool of 8 native halophytes. The random draws of 3-species and 6-species assemblages produced approximately equal numbers of plants per species for the experiment as a whole, yet only three species recruited> 10 seedlings per plot.Salicornia virginica andSalicornia bigelovii each produced> 15,000 seedlings in 1998, andSuaeda esteroa produced> 2,500 seedlings in 1999. For these 3 species, seedling recruitment increased with elevation in 1998, but this trend weakened in 1999, when species richness affected recruitment (fewer seedlings in more species-rich plots). Abiotic effects preceded biotic interactions in determining seedling recruitment patterns early in the development of the salt marsh. Effects of species richness appeared to be scale-dependent in that having all species present in the site likely enhanced overall recruitment (all species had 2 or more seedlings), while plantings of 6 species in a 2 × 2 m plot reduced seedling density.S. virginica was the only species that increased its presence and relative cover in the experimental site over the 4-yr study. Protocols for planting southern California salt marsh restoration sites could omit this species, but all others probably need to be introduced to restore diverse vegetation.

Vertical Accretion Rates and Heavy Metal Chronologies in Wetland Sediments of the Tijuana Estuary

This study represents the first report on sediment accretion rates using137Cs dating for a southern California salt marsh. Vertical accretion rates ranged from 0.7 to 1.2 cm yr−1, which is at the high end of sediment accretion values for coastal wetlands. This has lead to increases in elevation within the estuary from 18 to 35 cm over the last 35 years. Depth profiles of metal concentrations were converted to time-based profiles using vertical accretion rates. Chronologies for most cores indicate a consistent peak in sediment lead (Pb) concentrations in the early to mid 1980s, corresponding to the historic decline in Pb use, which was completed in the U.S. by the early 1980s, but not begun in Mexico until 1991. Sediment Pb levels ranged from about 6–56 μg g−1. Other metals did not show any consistent trends in sediment chronology, except for a single core from a mid-marsh site (east-mid 2), which showed a 2–3-fold increase in levels of Cu, Ni, and Zn during the past two decades. Sediment levels of copper (Cu), nickel (Ni), and zinc (Zn) ranged from 6–34 μg g−1, 11–27 μg g−1, and 42–122 μg g−1, respectively. Despite rapid industrial development of the watershed, a comparison of the sediment metal concentrations in the Tijuana Estuary to other anthropogenically-impacted estuaries in the United States and Europe, shows that metal levels in sediments of the north arm of the estuary are relatively low.

Salicornia virginica in a southern California salt marsh: Seasonal patterns and a nutrient-enrichment experiment

Salicornia virginica(common pickleweed) is the dominant vascular plant of many saline marshes of the US west coast, yet little is known about seasonal patterns or abiotic factors controlling it. In a southern California salt marsh, quarterly sampling revealed strong seasonal trends, with 2x greater S. virginica biomass in summer than in winter. Tissue nitrogen (N) and phosphorus (P) concentrations were highest in winter and lower in spring and summer, suggesting a dilution of nutrients as plants accumulated biomass during the growing season. Despite high sediment nutrient levels in this marsh, an experiment examining N and P effects still found strong S. virginica responses to N applied biweekly for .1 year. Increases in succulent tissue biomass after N addition were first seen in April 1998 (after fertilization for 11 months); two-fold increases in biomass and the number of branches resulted by the end of the experiment in August 1998. Addition of N increased N concentration in the woody tissues when sampled in August. The N:P ratio increased with N addition beginning in winter (7 months after fertilization began) and continuing through the remainder of the experiment. Effects of P addition were less marked, as adding P did not result in biomass responses; however, it did influence tissue nutrient levels. These amendments increased P concen- trations in the woody tissue in August 1998. In contrast to N amendments, which did not affect root nutrient concentrations, P addition led to increases in P content of root tissues in the latter portion of the growing season. These data suggest that increases in nutrients (especially N, but also P) can lead to large changes in S. virginica characteristics even in estuaries with high sediment nutrient levels.

Fish use of tidal creek habitats in two southern California salt marshes

Fish assemblages from small intertidal creeks (first-order) and from larger subtidal creeks (third- and fourth-order) were sampled seasonally for 1 year at Tijuana Estuary and Sweetwater Marsh in southern California, USA. First-order creeks were dominated by Gillichthys mirabilis, Fundulus parvipinnis, and Clevelandia ios at both sites. At Sweetwater Marsh, species composition and species richness differed between creek orders, as did size structure of F. parvipinnis, one of the most abundant marsh residents; a greater proportion of small juveniles was collected from first-order creeks. At Tijuana Estuary, the larger system, elevations of first-order creeks were higher and fish use of creeks was lower than at Sweetwater Marsh. Shallow habitats are used extensively by several marsh residents, and may provide nursery habitat for juvenile F. parvipinnis. Intertidal creeks should be considered in designs of marsh restoration projects where the creation of habitat for resident fishes is a priority.

Developing an indicator of nutrient enrichment in coastal estuaries and lagoons using tissue nitrogen content of the opportunistic alga, Enteromorpha intestinalis (L. Link)

We explored the use of an opportunistic green alga, Enteromorpha intestinalis (L. Link), as an indicator of N enrichment in a southern California salt marsh. In conjunction with N additions to cordgrass ( Spartina foliosa, Trin) in April, June and August 1995, mesh bags containing N-starved algal tissue were placed within cordgrass patches, at their edges along islands, and in adjacent channels. After 1 week in the field, recovered algal tissue was used to test detection of two levels of total N supply (one twice as high as the other), as well as no added N (control). Tissue N concentration, calculated as the percentage change in N, was the best of several algal measures at discerning differences in N availability in any month. In both April and June, tissue N declined from the marsh plain to the channels, reflecting declining N supply. Tissue N concentration also reflected differences in the total quantity of N added. Within the channels adjacent to fertilized areas, algal tissue N was similar to control areas, suggesting that N additions to cordgrass are not resulting in eutrophication of open waters. In August, the algae detected N additions on the marsh plain, but survivorship was poor; other algal species may be better indicators of enrichment in late-summer. With further investigation, the technique presented in this paper has the potential to be developed into a useful bioassay for detecting eutrophication of coastal salt marshes and lagoons.

Importance of Vascular Plant and Algal Production to Macro-invertebrate Consumers in a Southern California Salt Marsh

The dietary importance of marsh vascular plants (primarilySalicornia virginica), algae and upland particulate inputs to macro-invertebrate consumers was studied in Carpinteria Salt Marsh, southern California, using stable carbon and nitrogen isotope ratios. This marsh is predominantly a marine or hypersaline system and succulents are the most common vascular plant species. Of invertebrates collected from the vegetated marsh, tidal flats and channels, only detritivores from the vegetated marsh (Traskorchestia traskiana,Melampus olivaceus) had isotope values (δ13C=−20‰) that suggested some use ofSalicornia-derived carbon.T. traskianacultured in the laboratory on decomposingS. virginicaor blue-green micro-algal mat had distinctive isotopic signatures, reflecting the capability of this consumer to assimilate carbon and nitrogen derived from these sources. The δ13C values (generally −16‰ to −15‰) of species from tidal flats and channels (e.g.Cerithidea californica,Protothaca staminea,Mytilus galloprovincialis,Neotrypaea californiensis) were most similar to values for benthic algae and phytoplankton. Specimens ofM. galloprovincialisalong a gradient of presumed increase in marine influence had similar isotope values, suggesting little contribution to diet from upland runoff. The present results differ most noticeably from published values in the13C enrichment of suspension-feeders, suggesting the use of resuspended13C-enriched benthic microalgae in tidal channels by these consumers, and in the13C depletion and15N enrichment of plants and consumers along a portion of the marsh boundary receiving inputs of nutrient-enriched perched groundwater. In general, the isotopic composition of macro-invertebrates indicated the incorporation of algal production rather than ofS. virginicaor upland sources into the marsh food web.

Differential Effects of Salinity and Soil Saturation on Native and Exotic Plants of a Coastal Salt Marsh

In many southern California salt marshes, increased freshwater inflows have promoted the establishment of exotic plant species. A comparative study showed that a native, perennial, high marsh dominant,Salicornia subterminalis, and an invasive, exotic annual grass,Polypogon monspeliensis, responded differently to soil salinity and saturation.Salicornia subterminalis seeds and young plants were more salt tolerant, and the native grew best at high salinities (23 g 1−1 and 34 g 1−1) in greenhouse experiments. In contrast, the exotic had reduced growth at high salinities relative to nonsaline controls. The native,S. subterminalis, grew poorly as the duration of soil saturation increased from 2 wk to 32 wk, butP. monspeliensis grew equally well for all durations tested. The response ofS. subterminalis andP. monspeliensis to increased salinity indicated that salt applications might be used to protect native vegetation in salt marshes where salt-sensitive exotics are a problem. A field experiment verified that a salt application of 850 g m−2 mo−1 for 3 mo was sufficient to control the exotic, while not noticeably affecting the native. Thus, salt applications may be a practical method for controllingP. monspeliensis invasions in areas receiving urban runoff or other unwanted freshwater inflows.

Interactions between a salt marsh native perennial (Salicornia virginica) and an exotic annual (Polypogon monspeliensis) under varied salinity and hydroperiod

An exotic grass invades salt marshes of southern California in very wet years and where there are sewage spills or urban runoff. A series of growth-chamber, mesocosm, and greenhouse experiments explored whether soil salinity and/or waterlogging could explain invasion patterns. In all experiments, salinity significantly affected the growth and distribution of Polypogon monspeliensis (rabbit-foot grass, an exotic annual grass) and Salicornia virginica (pickleweed, a native perennial succulent). High salinities caused a greater reduction in seed germination rates for P. monspeliensis than for S. virginica, indicating that high salinity limits establishment and the spread of this exotic grass. At Tijuana Estuary, fresh water inputs to tidal mesocosms lowered soil salinities, increased cover of P. monspeliensis, and decreased cover of S. virginica. Polypogon monspeliensis outcompeted S. virginica under all salinity and hydrology treatments in the greenhouse experiment. Seasonally-low soil salinities caused by winter runoff and anthropogenic fresh water inputs are the likely factors controlling annual variations in the distribution of P. monspeliensis in southern California salt marshes. Our understanding of the causes of invasion is readily applicable to management: local invasions may be reversed by adding salt, and larger scale problems could be avoided by reinstating more natural hydrologic regimes.

RAPD analysis reveals low genetic variability in the endangered light‐footed clapper rail

Numbers of light-footed clapper rails Rallus longirostris levipes, an endangered bird inhabiting southern California salt marshes, have substantially declined from historic levels. RAPD (randomly amplified polymorphic DNA) analysis was employed to assess the genetic variability within and among four of the largest remaining light-footed clapper rail populations. A single, larger population of the endangered Yuma clapper rail Rallus longirostris yumanensis was used for comparison. A total of 325 RAPD primers were tested on DNA from a subset of five clapper rails composed of a single representative for each of the four light-footed clapper rail populations and a representative for the single Yuma clapper rail population. Of the 1338 amplified bands (loci) surveyed in these five representative birds, approximately 1% were polymorphic, indicating the level of differentiation across all loci is quite low. Nine primers yielding these 16 polymorphic bands were used to analyse 48 individuals from five populations. Five of these bands were polymorphic in both subspecies, six were polymorphic only within the light-footed clapper rails, and five were polymorphic only within the Yuma clapper rail samples. Considering the few bands that were polymorphic among the light-footed clapper rail populations, a surprisingly high level of population differentiation (GST = 0.28) was found. This is in accord with the results of AMOVA analyses which show that a fairly high percentage of the limited variability among the rails is due to either differences between subspecies or differences between the light-footed rail populations. Because inbreeding depression is suspected and overall genetic distances between populations are low, movement of light-footed clapper rails from larger populations into smaller ones might be considered as a management strategy. Employing RAPDs as one of a series of assays is useful in revealing the population structure of genetically depauperate species.

RAPD analysis reveals low genetic variability in the endangered light‐footed clapper rail

Numbers of light‐footed clapper rails Rallus longirostris levipes, an endangered bird inhabiting southern California salt marshes, have substantially declined from historic levels. RAPD (randomly amplified polymorphic DNA) analysis was employed to assess the genetic variability within and among four of the largest remaining light‐footed clapper rail populations. A single, larger population of the endangered Yuma clapper rail Rallus longirostris yumanensis was used for comparison.A total of 325 RAPD primers were tested on DNA from a subset of five clapper rails composed of a single representative for each of the four light‐footed clapper rail populations and a representative for the single Yuma clapper rail population. Of the 1338 amplified bands (loci) surveyed in these five representative birds, approximately 1% were polymorphic, indicating the level of differentiation across all loci is quite low. Nine primers yielding these 16 polymorphic bands were used to analyse 48 individuals from five populations. Five of these bands were polymorphic in both subspecies, six were polymorphic only within the light‐footed clapper rails, and five were polymorphic only within the Yuma clapper rail samples. Considering the few bands that were polymorphic among the light‐footed clapper rail populations, a surprisingly high level of population differentiation (GST= 0.28) was found. This is in accord with the results of AMOVA analyses which show that a fairly high percentage of the limited variability among the rails is due to either differences between subspecies or differences between the light‐footed rail populations. Because inbreeding depression is suspected and overall genetic distances between populations are low, movement of light‐footed clapper rails from larger populations into smaller ones might be considered as a management strategy. Employing RAPDs as one of a series of assays is useful in revealing the population structure of genetically depauperate species.

Influence of Watershed Runoff on Nutrient Dynamics in a Southern California Salt Marsh

The effect of freshwater runoff on nutrient loading and dynamics was studied in Carpinteria Salt Marsh, a typically small (93 ha) southern California, U.S.A., estuary adjoining a watershed supporting extensive agricultural and urban development. Concentrations of dissolved nitrate, but not ammonium or phosphate, were elevated in stream flow and perched groundwater in the watershed, relative to marsh surface waters. Loading of nitrate and particulate organic nitrogen (PON) from the watershed, but not ammonium or phosphate, increased as a function of stream discharge. Estimates of net nutrient exchange over selected tidal cycles suggested that the marsh exported nitrate, ammonium and PON to the Santa Barbara Channel. Nitrate and PON were advected from the watershed through the marsh while ammonium was produced within the marsh and exported. The availability of imported nitrogen for primary production may be limited by marsh tidal elevation as most nitrogen inputs are throughput to the Santa Barbara Channel.

Simulation model evaluation of potential recovery of endangered light-footed clapper rail populations

Conservation of the endangered light-footed clapper rail Rallus longirosris levipes is a major objective in the remaining areas of Southern California salt marsh. We used simulation modeling to determine the potential of different types of estuaries and different management strategies for increasing the size of the rail population. Analyses of existing data show that the size of breeding populations of rails is closely associated with above-ground biomass of Pacific cordgrass Spartina foliosa. A model cordgrass was constructed to analyze the effects of salinity and soil moisture, and of the impact of various management actions, on the potential for biomass production. The model shows that keeping the salt marsh open to the sea is critically important, but that enhancing the fresh water supplied to the marsh has mixed results. The simulation model was used in conjunction with field estimates of rail density to estimate the potential for rail population recovery. These estimates show that full implementation of the recovery plan for rails is only possible if all remaining wetland area is restored to high-quality rail habitat. Our results suggest that a greater benefit to rails may result from concentrating on habitat improvement of those estuaries that have the greatest potential for Spartina production rather than the dispersion of expenditures upon all existing wetlands, many of which may have limited potential for rail population expansion.

Freshwater Impacts in Normally Hypersaline Marshes

Heavy rainfall in 1978 and 1980 caused flooding of southern California salt marshes. Examination of three marshes demonstrated a broad range of freshwater effects which correlated with the degree of change in soil salinity. At Tijuana Estuary (1980), a short-term reduction in the salinity of normally hypersaline soils was followed by a 40% increase in the August biomass of Spartina foliosa. At Los Penasquitos Lagoon (1978), a longer period of brackish water influence was followed by a 160% increase in August biomass of Salicornia virginica. At the San Diego River (1980), flood flows were augmented by major reservoir discharge. Continuous freshwater flow leached most of the marsh soil salts and caused replacement of halophytes by freshwater marsh species. The first two cases probably fell within the normal range of flooding events, even though the hydrology of both watersheds has been modified. The vegetation response was functional; productivity increased but there was no major change in species composition. As expected, vegetation rapidly returned to preflood conditions. However, the long-term freshwater flow in the Dan Diego River was unnatural. Floral composition changed as soils were leached of salts. Recovery following the return of saline soils has been slow because many native halophytes are not good colonizers. The system's resilience is limited, and modification of natural stream discharge can cause permanent changes in coastal wetlands.

Salt marsh productivity with natural and altered tidal circulation.

The effects of altered tidal circulation on southern California salt marshes are investigated by comparing a well-flushed wetland and two modified wetlands which have reduced tidal flow. The Tijuana Estuary had continuous exchange of seawater but relatively low net aerial primary productivity (0.4-1.0 kg m-2yr-1) of vascular plants. Low productivity (0.6 kg m-2yr-1) was also found in the Flood Control Channel of the San Diego River, where tidal exchange was restricted to flow through a riprap dike. High productivity (1.2-2.9 kg m-2yr-1) in Los Penasquitos Lagoon was attributed to the influences of freshwater impounded behind a sand bar which blocked the mouth of the lagoon during much of the study period.It is hypothesized that elimination of tidal flow during the growing season increased primary productivity of vascular plants because freshwater runoff decreased soil salinity and because nutrients were retained within the marsh. However, we predict that sand bar obstruction can decrease productivity if below-average rainfall leads to hypersalinity of closed lagoons. Comprehensive evaluation of the effects of altered tidal circulation requires longterm study and examination of the total ecosystem.