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Christopher P. Onuf

Publications and source records attributed to Christopher P. Onuf.

12 recordsLinked to original sources
USGS2007

A GIS Analysis of Seagrass Resources and Condition Within Padre Island National Seashore, Texas

A survey of the seagrass resources of Padre Island National Seashore was conducted in fall 2002 and 2003, with additional sampling through 2006, to resolve distribution questions. Location coordinates were recorded to thousandths of minutes of latitude and longitude and converted to decimal degrees (minus decimal degrees for longitude) for import into ArcView (Environmental Systems Research Institute, Inc.). The seagrass core frequency data were developed as a theme in ArcView and overlaid on digital orthophoto quarter quadrangles of the U.S. Geological Survey to show sample depth with respect to mean sea level and frequency of occurrence of seagrass for five samples collected from every station sampled. These data were used to draw boundaries of area submerged at mean sea level and seagrass meadow in relation to the boundary of Padre Island National Seashore. Frequency of seagrass occurrence, mean plant height, shoot density, plant height multiplied by shoot density, live biomass, and dead biomass on a 1' latitude by 0.25' longitude grid were collected, and their distribution was plotted in space and according to depth. A User Guide for displaying data in ArcView is included at the end of this report. Seagrasses covered almost two-thirds of the regularly flooded part of Laguna Madre within the borders of Padre Island National Seashore. Comparisons with earlier surveys showed that substantial areas of seagrass cover had been lost in deep water between 1988 and 1998 as a result of a persistent phytoplankton bloom, and little recovery has occurred since. Maximum depth of seagrass occurrence responded to changes in water clarity. In contrast, much of the cover at shallow to intermediate depths lost at the south end of the study area between 1988 and 1998 was replaced by 2003. The seven stations with greatest plant height were located in this area of recent recolonization. Continuity of cover as measured by frequency of occurrence was high except near the edge of seagrass meadow. Decrease in this measure may be an indicator of meadow fragmentation, signaling deterioration of seagrass meadow before loss. The other measures of condition were so variable that they were insensitive indicators of impending change.

Open-File Report↗
USGS2002

Carbon budget for a subtropical seagrass dominated coastal lagoon: How important are seagrasses to total ecosystem net primary production?

It has been assumed that because seagrasses dominate macrophyte biomass in many estuaries they also dominate primary production. We tested this assumption by developing three carbon budgets to examine the contribution of autotrophic components to the total ecosystem net primary production (TENPP) of Lower Laguna Madre, Texas. The first budget coupled average photosynthetic parameters with average daily irradiance to calculate daily production. The second budget used average photosynthetic parameters and hourly in situ irradiance to estimate productivity. The third budget integrated temperature-adjusted photosynthetic parameters (using Q 10 =2) and hourly in situ irradiance to estimate productivity. For each budget TENPP was calculated by integrating production from each autotroph based on the producers’ areal distribution within the entire Lower Laguna Madre. All budgets indicated that macroalgae account for 33–42% of TENPP and seagrasses consistently accounted for about 33–38%. The contribution by phytoplankton was consistently about 15–20%, and the contribution from the benthic microalgae varied between 8% and 36% of TENPP, although this may have been underestimated due to our exclusion of the within bed microphytobenthos component. The water column over the seagrass beds was net heterotrophic and consequently was a carbon sink consuming between 5% and 22% of TENPP, TENPP ranged between 5.41×10 10 and 2.53×10 11 g C yr −1 , depending on which budget was used. The simplest, most idealized budget predicted the highest TENPP, while the more realistic budgets predicted lower values. Annual production rates estimated using the third budget for Halodule urightii and Thalassia testudinum compare well with field data. Macroalgae and microalgae contribute 50–60% of TENPP, and seagrass may be more important as three-dimensional habitat (i.e., structure) than as a source of organic carbon to the water column in Lower Laguna Madre.

Texas↗
USGS1996

Inexpensive, easy-to-construct suction coring devices usable from small boats

Collection of sediment cores in depths of 1-5 m is difficult with traditional sampling gear. Here we describe three suction coring devices constructed with readily available plumbing supplies and parts easily made from acrylic plastic and silicone sealant. The samplers have been used successfully in sediments ranging from coarse sands and shell hash to muds, highly organic deposits, and dense clays. Successful applications have ranged from contaminants analysis, toxicity testing, seagrass mapping, and assessment of sediment-microfloral interactions to sampling the infauna of surf-swept beaches.

Journal of Sedimentary Research↗
USGS1996

Seagrass responses to long-term light reduction by brown tide in upper Laguna Madre, Texas: Distribution and biomass patterns

A brown tide caused by a very dense bloom of an as yet undescribed species of the new class Pelagophyceae was first reported in upper Laguna Madre, Texas, USA, in June 1990 and has been there continuously through December 1995. No change in response to reduced light was evident in the distribution of the seagrass Halodule wrightii along transects sampled before the brown tide in 1988 and resampled after initiation of the brown tide in 1991 and 1992; however, in winter 1993-94 losses were documented over 2.6 km 2 of bottom and by winter 1994-95 the area of vegetation lost had more than tripled to 9.4 km 2 . Changes in biomass presaged the changes in distribution. Decreases in biomass at depths >1.4 m were evident 2 yr before bare areas were detected. Reductions in biomass were more pronounced toward the south, in keeping with a gradient of increasing light attenuation from north to south. Support of a diminishing number of new shoots by reclamation of nutrients and stored reserves from senescing shoots and rhizomes may allow H. wrightii to persist under conditions of insufficient light for periods greatly in excess of the life span of any one shoot. This postulated capability would account for the pattern of diminishing biomass over time where the seagrass persists in deeper areas and the long lag between light reduction and change in distribution where the seagrass succumbed.

Texas↗
USGS1996

Biomass patterns in seagrass meadows of the Laguna Madre, Texas

The Laguna Madre of Texas supports the most extensive seagrass meadows in the western Gulf of Mexico, In 1988 seagrasses covered 730 km 2 or about three-quarters of the embayment. Halodule wrightii dominated the entire upper laguna, and total biomass was quite uniform near 160 g˙m -2 throughout. Four species shared dominance in the lower laguna. Where present mean biomass of Thalassia testudinum was 373 g˙m -2 ; Syringodium filiforme , 138 g˙m -2 ; Halodule wrightii , 78 g˙m -2 ; and Halophila engelmannii , 6 g˙m -2 . Macroalgae were widespread, at 71 g˙m -2 where present. Halodule wrightii was dominant and biomass was low on sand flats of the barrier island separating the laguna from the gulf. Halophila engelmannii was limited to the deep edges of meadows. Biomass >300 g˙m -2 was limited to one extensive T. testudinum meadow at the south end of the laguna at all vegetated depths and two small areas with S. filiforme dominant at intermediate depths. Laguna Madre was similar in biomass to the two regions of extensive development of seagrasses in the eastern gulf: the Big Bend region of Florida (Iverson and Bittaker, 1986) and Florida Bay (Iverson and Bittaker, 1986; Zieman, et al., 1989). The laguna differed from the eastern gulf sites in more turbid waters and much shallower maximum depths of occurrence of seagrasses (<2 m compared to 8–11 m), higher contribution of H. wrightii to system-wide biomass (although H. wrightii 's share has rapidly diminished in Laguna Madre as it has been displaced by S. filiforme and T. testudinum [Quammen and Onuf, 1993]), and much higher macroalgal biomass (limited to the section of the lower laguna receiving agricultural inflows). Similarities in the environments of Laguna Madre and inner Florida Bay suggest that, if cover and biomass of Thalassia continue to increase, the laguna may become vulnerable to mass mortalities as are now occurring in Florida Bay.

Texas↗
USGS1994

Seagrasses, dredging and light in Laguna Madre, Texas, U.S.A.

Light reduction resulting from maintenance dredging was the suspected cause of large-scale loss of seagrass cover in deep parts of Laguna Madre between surveys conducted in 1965 and 1974. Additional changes to 1988, together with an analysis of dredging frequency and intensity for different parts of the laguna, were consistent with this interpretation. Intensive monitoring of the underwater light regime and compilation of detailed environmental data for 3 months before and 15 months after a dredging project in 1988 revealed reduced light attributable to dredging in four of eight subdivisions of the study area, including the most extensive seagrass meadow in the study area. Dredging effects were strongest close to disposal areas used during this project but still were detectable on transects >1·2 km from the nearest dredge disposal area. In the subdivision of the study area where most of the dredge disposal occurred, light attenuation was increased throughout the 15 months of observation after dredging. In the seagrass meadow and the transition zone at the outer edge of the meadow, effects were evident up to 10 months after dredging. Resuspension and dispersion events caused by wind-generated waves are responsible for the propagation of dredge-related turbidity over space and time in this system.

Texas↗
USGS1994

Distribution and status of submerged vegetation in estuaries of the upper Texas coast

Composition and biomass of beds of submerged aquatic vegetation (SAV) in estuaries along the upper Texas coast were examined from bottom core and rake samples taken along 72 line transects during July–August 1987. Substrate composition, salinity, water depth, and water clarity were also measured at each sample station. Halodule wrightii was the dominant species of SAV meadows (69% occurrence), followed by Ruppia maritima (36%). Najas guadalupensis and Vallisneria americana were only found in the shallow (<60 cm), oligohaline (<10 ppt) waters of Trinity Bay. Halophila engelmannii and Thalassia testudinum were only found in the deeper (35–110 cm) euhaline waters (30–40 ppt) of Christmas and Matagorda Bays. H. wrightii and R. maritima occurred across the greatest depth and salinity spectra, with R. maritima more prominent in shallower (<90 cm) mixohaline waters (10–30 ppt) and H. wrightii more prominent in euhaline waters (30–40 ppt). Beds of SAV were more extensive and had greater biomass along south shorelines compared to north shorelines (P<0.045), and depth to which vegetation grew was positively correlated with water clarity (r=0.927, P<0.001). The correlations of water depth, salinity, and water clarity with variation in SAV species composition and distribution suggest that abiotic tolerances documented by previous experimental studies are major factors influencing field distributions. Comparisons with similar field data from the early 1970s corroborate the reported decrease in SAV in the Galveston Bay complex and suggest a slight increase in the Matagorda Bay complex. Decline of SAV in the Galveston Bay complex coincided with major shorefront development and a decline in numbers of wintering redhead ducks Aythya americana . Protection of shorelines of upper coast estuaries is vital to maintaining SAV resources and the dependent fauna. Where possible, barrier island preserves should be established adjacent to beds of-SAV and development restrictions implemented to reduce impacts.

Texas↗
USGS1993

Laguna madre: Seagrass changes continue decades after salinity reduction

Vegetation maps of the lower Laguna Madre prepared from surveys conducted in 1965–1967, 1974–1976, and 1988 document a >330 km 2 decrease in cover by Halodule wrightii , an increase of almost 190 km 2 in other seagrass species, and an increase of 140 km 2 in bare bottom. Loss in seagrass cover is confined to deeper parts of the laguna; turbidity caused by maintenance dredging is the suspected cause. The species shifts are consistent with observed reductions in salinity maxima. Although the hydrological alterations and climatic shift responsible for moderating the salinity regime occurred between 1948 and 1965, the biological changes continue. Establishment of patches away from source meadows appears to be the process for displacing species that limits their rate of expansion into suitable habitat in this elongate embayment.

Texas↗
USGS1988

Habitat Suitability Index Models: Red king crab

A review and synthesis of existing information were used to develop a Habitat Suitability Index (HSI) model for evaluating habitat of different life stages of red king crab (Paralithodes camtschatica). A model consolidates habitat use information into a framework appropriate for field application, and is scaled to produce an index between 0.0 (unsuitable habitat) and 1.0 (optimum habitat) in Alaskan coastal waters, especially in the Gulf of Alaska and the southeastern Bering Sea. HSI models are designed to be used with Habitat Evaluation Procedures previously developed by the U.S. Fish and Wildlife Service.

FWS/OBS↗
USGS1982

Habitat Suitability Index Models: Juvenile Atlantic Croaker

INTRODUCTION The Atlantic croaker is an important commercial and recreational species. In the 1940's, the foodfish catch of Atlantic croakers was concentrated in Chesapeake Bay; in the 1950's and early 1970's, the catch was concentrated in the Gulf of Mexico; and in the late 1970's, the catch was concentrated in the South Atlantic States (Wilk 1981). Industrial and recreational catches of Atlantic croakers have been concentrated in the Gulf of Mexico, where the Atlantic croaker is the most important species of bottomfish for industrial uses (Knudsen and Herke 1978), and has ranked first, second, or third in number caught by recreational anglers, depending on survey year (Nakamura 1981). Today, Virginia or Delaware is considered to be the northern extent of the species. During climatically warmer periods, such as the 1930's and 1940's, the croaker extended its range north at least to New York, where it was commercially fished. The southern extent of its range is Argentina.

FWS/OBS↗
USGS1977

Interactions of nutrients, plant growth and herbivory in a mangrove ecosystem

The effect of nutrient enrichment of red mangroves ( Rhizophora mangle ) was studied by comparing two mangrove—covered islands in the Indian River at Ft. Pierce, Florida, USA, one (high nutrient) with and one (low nutrient) without a breeding colony of pelicans and egrets. Repeated measurements taken on > 100 tagged branches in each area revealed higher growth rates at the high nutrient site. Trees at the high nutrient site showed greater additions of (1) leaves; (2) reproductive parts; (3) new lateral branches; and (4) larger increments to existing stems. Growth in the fertilized stand also began earlier in the year and had a second maximum not shared by the low nutrient area. Both leaves and fruits at the high nutrient site were richer in nitrogen. More striking than the effects on plant growth, however, was the proportionately much greater stimulation of herbivory by insects in response to nutrient enrichment. Larvae of the five lepidopteran species that we observed feeding on buds or leaves were either more abundant or only present in the high nutrient area, as was the scolytid beetle that infested seedlings before they dropped from the parent tree. This difference in herbivory between sites disappeared when the birds seasonally migrated away from their nesting areas at the high nutrient site. This observation and the demonstration that the mangrove skipper Phocides pigmalion attains a higher growth efficiency on high nutrient leaves are both consistent with the hypothesis that increased nutritive value of vegetation (correlated with the increased concentration of nitrogen) is responsible for the 4x greater losses to herbivores in the high nutrient. Implications for environmental management and in more complex communities are discussed.

Florida↗