The Ecology of Tijuana Estuary, California: An Estuarine Profile

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,1979 1980 (Flood Year) 1981 W . - 7 t- 1982 (No Data) 1983 (Drought Year) also vary. However, the combination of severe environmental stresses, such as drought combined with estuary closure, is probably rare. Thus, local extinction is not viewed as a common event for this estuary. Species richness in the region's wetlands is high for systems with good tidal flushing and low for systems that frequently close to tides (Zedler 1982b); the near loss of annual pickleweed and sea-blite was no doubt due to a combination of events. Whether or not either species will recover depends in part on management of the estuary. The dredging that preceded reopening of the estuary mouth in December 1984 appears to have increased tidal flushing beyond what it was in 1983. If tidal flushing is greatly improved after pickleweed has become dominant, this may well be detrimental to recovery of short-lived species. Without tidal action, soils dry in summer, and perennial pickleweed grows very well (78% cover in 1984). With sluggish tides, drainage of the intertidal marsh is incomplete, soils remain saturated for long periods of time, and perennial pickleweed is at a disadvantage. These were the conditions of 1983, when pickleweed cover was at a low of 38% (Table 26). With maximal tidal flow, the marsh is periodically well-drained, and pickleweed cover may remain high (as in 1985). If these conditions persist, competitive dominance by perennial pickleweed is expected to preclude reestablishment of annual pickleweed and sea-blite and to reduce the ability of cordgrass to recover. This hypothesis is being tested with manipulative experiments at Tijuana Estuary. Figure 63. Changes in pickleweed distribution along eight transects (see Figure 60; data are explained in Figure 62). See Table 26 for changes in frequency and percent cover. These hypotheses of reduced seed bank and competitive effects of pickleweed are being tested experimentally. It is likely that expansfon and shrinkage of spectes drstrrbuttons is the norm for the region's highly dynamic wetlands. The estuary's history of variable rainfall and streamflow, fluctuating sea levels, and alternating conditions of good and slugg~sh tidal flow, all suggest that the abundance and distributional limits of marsh specles should 5.3.3 Summary of Compositional Changes The species characteristic of southern Californta coastal wetlands do not conform to the classical separatton of halophytes and glycophytes on the basrs of tolerance to 05 ppt salrnities (Waisel 1972) There is a spectrum of tolerances, and establishment is determined by the degree and duration of freshwater influence the low-salinlty gap Salinity determines germination, and durat~on of the required salinity and soil moisture conditions determines seedling survrval This is consistent with the regeneration niche concept of Grubb (19771, who hypothesized that multlple characterist~cs of both species and environment influence establishment Once established, salt marsh plant populations may persist or go extlnct, depend~ng on environmental condittons that may differ greatly from those controlling establrshment A conceptual model was developed to surnmarlse how low-sal~nity gaps control invasions and extreme stresses control extinctions (Figure 64) Cond~tions at both Tijuana Estuary and San D~ego R~ver were used in comparison wrtn changes follsw~ng wetter and drier condit~ons (Zedler and Beare, in press)
Table 27. Occurrence (in circular 0.25m2 quadrats) of the most frequent species of the Tijuana Estuary salt marsh, 1974 and 1984. Species Total Frequency Total Frequency occurrences t occurrencesC (ob Salrcornra v~rgrnrca pickleweed S brgelovrr annual p~ckleweed Jaumea carnosa Batrs mantrma saltwort Frankenla grand~folra alkali heath Monanthochloe Irttoralrs shore grass Suaeda esteroa sea-bllte Salrcornra subtermrnairs perennial glasswort Spartrna folrosa cordgrass Drstrchlrs sprcafa salt grass L~mon~um calrforn~cum sea lavender Trrgloch~n conclnnum arrow grass Cressa truxrllens~s alkali weed Cuscuta salrna dodder dData from 357 quadrats along 3 transects elevatcon range was 30-130 cm above MSL (Zedler 1977) b~ata from 216 quadrats elevat~on range was 50-180 crn MSi COccurrences cn noncordgrass transects t cordgrass transects total "pec~es that were nearly eiim~nated in 1984 a. With typical winter rainfall. brief sallnity reduction st~rnulates seed germination and allows seedling establishment Fresh-to-brackish marsh specles cannot invade during the narrow lowsalinity gap The fuli complement of salt marsh species persists rn the absence of extreme environmental stresses b. Extreme flooding reduces salinities substantially, cordgrass and other halophytic species can become established from seed However, because the low-salinity gap is brief, seedling establishment 1s limited to the normal complement of salt marsh species Short-term flooding does not appear to cause plant mortaiity. c If the low-salinity gap IS artificially prolonged, a var~ety of fresh and brackish marsh species can germinate and become established. if they grow to rhizome stage withln the gap, they may persist after hypersaline conditions return Specles that cannot tolerate prolonged inundation (I e , species with little aerenchyma) will undergo heavy mortality or go extinct Impoundment can eliminate salt marsh specles, such as pickleweed (Sal~cornra v~rglntca) Summer ~nundation events may be even more damaglng than winter In the Netherlands, experimental studtes of several halophytes indicated that warm temperatures reduce inundat~on tolerance (Groenendijk 1984)' thfs IS reasonable. since respiration rates would increase and growth would decline d. Without winter rainfail, seeds of only a few species may germinate, but seedllng establishment can be limited by drought and hypersalinity, especially when an estuary closes to tidal flushing. During dry years, most vegetat~vely-reproducing
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June 1986 I THE ECOLOGY OF TIJUANA
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The findings in this report are not
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CONVERSION TABLE Metric to U.S. Cus
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Page 4.5 Energy Flow ..............
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Number Page The channel fish cornrn
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ACKNOWLEDGMENTS The California Sea
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Figure 1. Lacatlcrn of 71juana Estu
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Figure 3. Map6 of Tijuana Estuary s
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FI(I(uPc$ 4. Wei&rahOt@ patt@rns of
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also limitrng Hot, dry desert winds
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2.3 LAND USE HlSPQWV Through exami!
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The nearby bridge remained, and ext
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flushing until mid-December 1984. E
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with seawater. Even in years with s
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flats ~n winter but not in midsumme
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Each "island" of higher topography
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The vegetation and soils of the upp
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populations (Massey 1979) The brrds
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c~n~entrations of insects, especial
Page 39 and 40:
Frgufe 29. The Ifght-footed clapper
Page 41 and 42: Figure 30. During winter (above) th
Page 43 and 44: anlmal footprints, In the hardpan f
Page 45 and 46: Tijuana Estuary salt marsh Such exp
Page 47 and 48: Frgura 36 The etraracrel an& tidal
Page 49 and 50: ^_ _I __"___I _-a_. -- fable 7. Inv
Page 51 and 52: Pre- 1980 (excluding Hosrner Rehse
Page 53 and 54: Table 8. Average density (number/m2
Page 55 and 56: changes in envlronment has not been
Page 57 and 58: was dominated by goby species, Cali
Page 59 and 60: compared the ,chthyoplankton commun
Page 61 and 62: Figure 43, Birds of the intertidal
Page 63 and 64: -" and that the community was compo
Page 65 and 66: Willet (1 73) i - Water Depth (cm)
Page 67 and 68: Table 14. Early information on dune
Page 69 and 70: Figure 48. Birds that nest on the s
Page 71 and 72: CHAPTER 4 ECOSYSTEM FldNCTlONlNG St
Page 73 and 74: were no? irght-saturated untri expo
Page 75 and 76: Table 17. Conceptual model of chann
Page 77 and 78: Table 19. Net aboveground productiv
Page 79 and 80: _I___ I__-~--- 4.4.1 Nitragen Fluxe
Page 81 and 82: i i t 1 1 I Table 22. Standing crop
Page 83 and 84: calculated from chlorophyll a measu
Page 85 and 86: Dissolved Organic Biomass Carbon De
Page 87 and 88: CHAPTER 5 THE ROLE OF DISTURBANCES
Page 89 and 90: Table 25. Changes in elevation in t
Page 91: Table 26. Change in pickleweed cano
Page 95 and 96: perennials can survive long periods
Page 97 and 98: An experiment was designed to deter
Page 99 and 100: CHAPTER 6 MANAGEMENT CONSIDERATIONS
Page 101 and 102: Closely associated with the researc
Page 103 and 104: c. Dredging at Tijuana Estuary will
Page 105 and 106: Table 30. Predicted impacts of redu
Page 107 and 108: footed clapper rails; the picklewee
Page 109 and 110: overwashas, and slabiiizatran rs rl
Page 112 and 113: REFERENCES Allen, L.G. 1980. Struct
Page 114 and 115: Minsky, D. 1984. California least t
Page 116: 30272 -101 REPORT WUMENTATION 1. RE
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The Ecology of Tijuana Estuary, California: An Estuarine Profile

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