Mexico were obtained from the Na- 

 tional Weather Service prior to sam- 

 pling and then adjusted to compensate 

 for the distance between the marsh 

 and the gulf. In an effort to con- 

 solidate the data for analysis the 

 mean and slack tides for each tide 

 cycle were combined. These events 

 were then designated as flood and ebb 

 tides. Exchanges of water between 

 the Nueces deltaic marsh and bay were 

 estimated by determining flow rates 

 past a sampling station. A mea- 

 sured distance of 100 ft (30.5 m) was 

 laid out along the bank of the Rincon 

 Bayou near its mouth. A weighted 

 (partially submerged) float was then 

 released in midstream and allowed to 

 move with tidal flows. Elapsed time 

 between start and finish yielded the 

 flow rate in m/sec. To determine 

 flow volumes, a cross-sectional map 

 of the bayou was made using a sound- 

 ing line and tide staff gauge (Figure 

 3). Individual measurements of velo- 

 city were multiplied by the cross- 

 sectional area represented by each 

 respective sampling point to estimate 

 volumes of flow. Velocity measure- 

 ments were replicated to ensure reli- 

 ability. 



ANALYTICAL METHODS AND 



EXCHANGE OF MATERIALS 



Total organic carbon and inor- 

 ganic carbon measurements were made 

 using a Dohrmann Model DC-50 organic 

 carbon analyzer modified to measure 

 inorganic carbon. Particulate orga- 

 nic carbon was determined using the 

 method described by Menzel and Vac- 

 caro (1964) and the Dohrmann DC-50 

 organic carbon analyzer. Total phos- 

 phorus and ortho-phosphorus concen- 

 trations were determined using the 

 oxidative and spectrophometric meth- 

 ods outlined in Standard Methods 

 (APHA 1975). Nitrate, nitrite, and 



organic nitrogen forms were deter- 

 mined using methods outlined in 

 Strickland and Parsons (1965) and 

 Standard Methods (APHA 1975). A 

 Beckman Model 25 UV-Vis Spectrophoto- 

 meter was used in the final analysis 

 of nitrogen and phosphorus forms. 

 Ammonia concentrations were deter- 

 mined using and Orion ammonia probe 

 and Model 901 Orion microprocessor. 

 The detection limits , of the probe 

 ranged from 0.02 x 10 to 1.7 x 10 

 mg/1 free ammonia. 



Exchanges of materials on each 

 sampling date were calculated by mul- 

 tiplying the mean concentrations of 

 replicate samples by the estimated 

 flow volume for each tide stage in a 

 24-hour tidal cycle. 



Total exchanges of nutrients 

 between the marsh and the estuary 

 were determined from the difference 

 in quantities of materials entering 

 (flood tides) and leaving (ebb tides) 

 the Rincon Bayou. 



RESULTS 



Flows in and out of the marsh 

 are for the most part attributable 

 to tidal action, and to a lesser ex- 

 tent wind seiche. Flow rates varied 

 seasonally with highest rates occur- 

 ring in the spring (8.61 m /sec in 

 April flood tide) and the lowest oc- 

 curring in the late fall (0.59 m / 

 sec in November flood tide). During 

 the course of the study no major wea- 

 ther events such as hurricanes or 

 sustained flooding occurred. Spills 

 from the Nueces River into the upper 

 marsh occurred only once during the 

 study period. This event was a brief 

 spill, which was dispersed by evapo- 

 ration and flows down tidal channels 

 through the marsh to Nueces Bay. Low- 

 er marsh inundation (Figure 2) oc- 

 curred daily during most of April and 

 May when tides were at their peak. 



476 



