Laguna Madre: Seagrass Status and Trends in the Northern Gulf of Mexico
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Laguna Madre By Christopher P. Onuf1 Background and precipitation so low that most of the watershed does not actually contribute to the lagoon (Texas Department of Laguna Madre is a place of superlatives. It is separated Water Resources, 1983). Consequently, there is little inflow from the Gulf of Mexico by Padre Island (fig. 1), the longest of nutrients or suspended particulates, and the waters of the barrier island in the world. Excluding its tributary Baffin Bay lagoon are generally clear. The only exception is Arroyo (not treated in this account), Laguna Madre makes up about Colorado, a distributary of the Rio Grande that is no longer 20% of Texas’ protected coastal waters while contributing connected to the river but serves as the major drain of the 40%–51% of the State’s commercial fish catch historically irrigated Lower Rio Grande Agricultural District and receives (Hedgpeth, 1947). The lagoon is even more important in the treated effluents from most of the rapidly growing support of the redhead duck, Aythya americana. Over 75% municipalities of the lower Rio Grande valley. Most of the of the world population of redhead ducks overwinters on the shoreline is undeveloped. Padre Island National Seashore Laguna Madre of Texas and on its sister lagoon, the Laguna and The Nature Conservancy control most of the east side Madre de Tamaulipas, just south of the delta of the Rio Grande of Laguna Madre, while most of the west side is owned by (Weller, 1964). While in residence, the redhead ducks feed large ranches, such as the King Ranch, and Laguna Atascosa almost exclusively on the rhizomes of one seagrass species, National Wildlife Refuge. Only at the northern and southern shoal grass (Halodule wrightii). The lagoon’s rich endowment ends is there considerable residential and marina development of seagrasses provides critical habitat for fish and waterfowl. of the landward margin of the lagoon. Row crop agriculture Seagrass meadows cover 65% of the bottom of Laguna Madre, occurs along intermittent creeks that drain into Baffin Bay. and Laguna Madre accounts for more than 75% of seagrass Although only limited development of the shoreline has cover along the entire Texas coast. The lagoon is also unusual occurred, some modifications of the lagoon itself are exerting for being one of only five hypersaline coastal ecosystems in a strong influence on the Laguna Madre ecosystem. Prior to the world (Javor, 1989). completion of the Gulf Intracoastal Waterway (GIWW) in 1949, a 20-km (12-mi) reach of emergent sand and mudflats had divided the lagoon into separate basins, which were then connected only under conditions of extreme high water. Scope of Area Now the GIWW provides a permanent water connection between upper Laguna Madre and lower Laguna Madre. This Laguna Madre is naturally divided into two segments (fig. hydrological connection, together with currents produced by 2) by a 20-km-long (12-mi-long) expanse of seldom-flooded prevailing winds oriented along the long axis of the lagoon, sand and mudflats. Upper Laguna Madre extends 80 km (50 improved circulation between basins and with the Gulf of mi) northward from the flats to its terminus in the southeast Mexico and moderated the extent to which lagoon waters corner of Corpus Christi Bay and ranges in width from 3 to 6 became hypersaline. km (~2 to 4 mi). Lower Laguna Madre extends 95 km (59 mi) southward from the flats to within 5 km (3 mi) of the Mexican border and ranges in width from 3 to 12 km (~2 to 7 mi). Both segments have been included in systematic mapping efforts Methodology Employed To Determine dating back to the 1960s. and Document Current Status A variety of circumstances act together to make the Laguna Madre hospitable environment for seagrasses. The Because of the particular importance of shoal grass for average depth of the lagoon is 1 m (3.3 ft). Although the redhead ducks wintering on Laguna Madre, the delineation mapped watershed of the lagoon is more than 15 times the of seagrass meadows according to dominant seagrass species water area of the lagoon, the surrounding land is so flat has been an essential element of seagrass mapping there. Accordingly, mapping efforts have built upon intensive field sampling along transects. Knowledge of current species 1 U.S. Geological Survey, National Wetlands Research Center. 30 Seagrass Status and Trends in the Northern Gulf of Mexico: 1940–2002 37 Corpus Corpus Christi Christi Bay e r d a M a n u g a L r e p p U Gulf of Mexico of Gulf Padre Island Padre Interstate highway River Gulf Intracoastal e r Waterway d a County line M a n u g a L r o e ad w r o lo o L Laguna Madre C McAllen o y o r Watershed boundary r A City Rio Gr N ande 20100km 4030 Brownsville 151050mi 20 25 Mexico Figure 1. Watershed for Laguna Madre. Status and Trends 31 distribution is based on a survey conducted mostly in 1998, or four 10-cm-diameter (4-inch-diameter) core samples plus but sampling efforts continued into 2000 to resolve questions material brought up on the anchor to determine the occurrence arising during production of the maps. of seagrasses at a location. In lower Laguna Madre, sampling was along east-west The source maps for the earlier surveys are small format transects at 2’ intervals of latitude from 26° 4’ N to 26° 48’ and have been digitized and transferred to a common scale for N. In upper Laguna Madre, transects generally conformed this analysis of trends. The shore is of such low relief along to those first used in a survey conducted in the mid-1970s much of Laguna Madre, particularly on the west side of Padre (Merkord, 1978) and resampled in 1988 (Quammen and Island, that the landward limits are poorly defined. No attempt Onuf, 1993). Spacing was irregular, but most transects were has been made to reconcile differences in shorelines between oriented perpendicular to the long axis of the lagoon and surveys. They either arise from differing interpretations of ranged from 1’ to 5’ of latitude apart. In 1998, transects were ambiguous features on aerial photographs or real differences added in the biggest gaps and eliminated where they were in water levels between mapping periods. These discrepancies closer than 2’ of latitude to provide more uniform coverage. led to large uncertainty in the extent of bare areas in the Along each transect, four 10-cm-diameter (4-inch-diameter) lagoon. Almost no dredge islands were shown on the 1960s cores were collected at stations located 0.25’ longitude apart maps, but they appear on USGS quadrangles from the same as determined by a Rockwell Precision Lightweight Global period. Little change was seen in these features between the Positioning System (GPS) Receiver with Federal Precise mid-1960s and the mid-1970s. Consequently, island polygons Positioning Service. Species of seagrass present in each core from the 1970s mapping were added to the 1960s map so and on the anchor were recorded in order of abundance, and that seagrass coverage estimates for the 1960s would not the station was classified according to dominant species. be artificially inflated. One area erroneously classified as Where cover type changed between stations, the boundary was “emergent and unvegetated submergent dredged material” established by sampling at successively smaller intervals until on the 1970s map was redrawn to show an area of different species were dominant or the presence or absence surviving seagrass. of seagrass coverage occurred at stations less than 0.01’ (17 No assessment of map accuracy was attempted. m) apart. Samples were not collected unless the GPS unit was Boundaries in the 1960s map could be off by as much as operating with a positional accuracy of less than 10 m (33 ft). 0.5 km (0.3 mi) because of the spacing of stations and by an The boundary coordinates of meadows and dominant unknown additional amount arising from errors in compass species were plotted on 25 U.S. Geological Survey (USGS) and dead-reckoning navigation. This uncertainty in seagrass 7.5’ topographic sheets representing the frequently submerged boundaries, however, is mitigated by the facts that seagrass parts of the lagoon. National Aerial Photography Program cover was continuous across the whole lagoon on many imagery flown in 1995 was projected onto the topographic transects and there was a very limited occurrence of any sheets to aid in interpolation of boundaries between transects. species except shoal grass. The resolution of the mapping Maps were sent to the USGS National Wetlands Research improved in successive surveys to 17 m (56 ft) on transects in Center in Lafayette, La., for digitization and geographic 1998, but with greater uncertainty for interpolated boundaries information system (GIS) analysis. between transects. North-south limits of seagrass cover were usually resolved to 0.01’ of latitude, but north-south limits between species were not resolved between transects in most Methodology Employed To Analyze cases. Because of these limits on the resolution of the maps, differences in seagrass coverage between surveys (decades) of Historical Trends less than 5 km2 (2 mi2) may not be indicative of change, unless the trend continues into subsequent surveys. As noted above, Whole lagoon surveys of seagrass distribution have been differences between surveys in what was considered “bare” conducted about every 10 yr since the mid-1960s. The 1960s bottom are even more uncertain. maps were taken from Federal aid projects conducted by the Texas Parks and Wildlife Department (TPWD) (McMahan, 1965–67), based on transects across the lagoon at about 1.6-km (1-mi) intervals with stations 0.5 km (0.3 mi) apart.