Ensenada de La Paz, Baja California Sur

by C.H. Lechuga-Devéze

Study Area Description

The Ensenada de La Paz (ELP, 24^° 14’ N, 110^° 29’ W), is an anti-estuarine coastal lagoon without freshwater inputs. There are no groundwater fluxes but saline intrusion towards underground freshwater reserves. Sewage is treated and used to irrigate small agricultural fields. About 150,000 people live adjacent to the main entrance of the lagoon in La Paz, but it is assumed that no sewage effluent flows directly into the lagoon. The total lagoon area is around 45 km ² (Lechuga-Devéze et al., 1986) with a depth of about 3 m and a total volume of 145 x 10 ⁶ m ³ (Gilmartin and Revelante, 1978). The climate is arid, with summer showers averaging 1.15 mm day ^-1 and highest evaporation averaging 5.71 mm day ^-1 (average for July to September). In the spring, the scarce rain averages 0.04 mm day ^-1 , and evaporation averages about 3.56 mm day ^-1 (average for March to May). During the 1970's, the natural shellfish banks (Argopecten circularis) inside the lagoon disappeared, and to-date no new populations have developed. Primary production of the system is about 1.2 g C m ^-2 day ^-1 (Lechuga-Devéze et al., 1986), that is, about 100 mmol m ^-2 day ^-1 . Some data have shown that the adjacent waters of the Bahía de La Paz are the main source of nitrate, phosphate and silicate for the lagoon (Lechuga-Devéze et al., 1986; Cervantes-Duarte, 1981), meaning that the lagoon does not support enough oxidative process to provide the inorganic nutrients for primary production. Salinity, nitrate and phosphate data gathered by our research group (Morquecho-Escamilla and Murillo-Murillo, 1995; Lechuga-Devéze et al., 1990) were used for the budgetary calculations to compare the summer to spring condition, representing the wet and dry season for the system.

Water and Salt Budgets

Figure 1 illustrates the water and salt budgets for this system for the spring, and the regional comparison tables summarize the data for both the spring and summer. The water exchange time for this system appears to be about a month.

Figure 1. Water and salt budgets for Ensenada de La Paz, spring. System volume is in units of 10 ³ m ³ . Water fluxes in 10 ³ m ³ day ^-1 . Salt fluxes in 10 ³ psu m ³ day ^-1 .

Budgets of Nonconservative Materials

P Balance

Figure 2 illustrates the P and N budgets for the spring, and the regional comparison tables summarize the data for both spring and summer. With particular respect to DDIP, there appears to be substantial variation; DIP uptake occurs and is higher during the summer than the spring. The average DDIP of the two sampling periods is –1,914 mol day ^-1 . Thus the system appears to be a net sink for DIP, mostly derived from the ocean.

N Balance

The bay appears to be a net DIN source during the spring and about neutral with respect to DIN during the summer. The average DDIN of the two sampling periods is +2,706 mol day ^-1 .

Stoichiometric Calculations of Aspects of Net System Metabolism

Nitrogen fixation minus denitrification (nfix-denit) is calculated from the difference between observed and expected DDIN, where the expected DDIN is calculated as 16 x DDIP. It is assumed that the major sink for DIP is plankton. Table 1 illustrates these calculations for Ensenada de La Paz. The system appears to fix nitrogen in excess of denitrification, by a rate averaging about 0.8 mmol m ^-2 day ^-1 .

Table 1. Estimated rates of nonconservative observed DIN fluxes and (nfix - denit) in Ensenada de La Paz.

The difference between organic carbon production, p, and respiration, r, within the system (NEM) is calculated using:

(p-r) = - DP x ( C:P)part

where (C:P)part represents the composition of particles reacting in the system. Taking the particle composition as the Redfield ratio (C:N:P = 106:16:1), since Ensenada de La Paz is assumed to be a plankton-dominated system, the estimates of DP and NEM are shown in Table 2:

Table 2. Estimated rates of nonconservative DIP fluxes and NEM in Ensenada de La Paz.

This system appears to be net autotrophic by approximately 5 mmol m ^-2 day ^-1 . Primary production has been estimated to be about 100 mmol m ^-2 day ^-1 , implying that respiration is about 95 and that the p/r ratio is about 1.05. This system appears to produce about 5% more organic carbon that it consumes.

Figure 2. DIP and DIN budgets for Ensenada de La Paz, spring. Fluxes in mol day ^-1.

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Last Updated 14 Jan 2000 by DPS