Bahía Concepción, Baja California Sur

by C.H. Lechuga-Devéze

Study Area Description

Bahía Concepción (BC, 26 ^o 30’ N, 111 ^o 30" W), the largest and deepest coastal embayment in the Gulf of California (282 km ² ; 4,553 x 10 ⁶ m ³ ), has insignificant human influences including scarce tourist facilities. Continental runoff is absent, although small hot springs are dispersed along the shoreline. The fluxes of these springs have not been studied; we assume that they are refluxing seawater. Two parts to the annual cycle are well identified. In winter (November-March), the bay is vertically well mixed. In summer, April-October, the central basin (33 m maximum depth) develops a strong thermocline that isolates the water below 20 m. This leads to anoxia and hydrogen sulphide production during August and September (Lechuga-Devéze et al., 1997; Reyes-Salinas, 1994). During this dystrophic period, high amounts of pigment are present, mainly chlorophyll b (Lechuga-Devéze, 1994). This indicates rapid metabolic cycling, perhaps chemoautotrophic. The water, salt, N, and P budgets developed here are only for the well-mixed period; the data were collected by our research group. It would also be useful to establish a separate set of budgets for the stratified period, although the long water exchange time (below) suggests that the budgets for the well-mixed period may characterise the annual average. For now, the annual summary budgets (regional comparison tables) are based on the data from this single period.

Water and Salt Budgets

There is no runoff and apparently no groundwater or other land-derived freshwater input; evaporation totals about 217 x 10 ³ m ³ day ^-1 , and precipitation totals about 65 x 10 ³ m ³ day ^-1 . Oceanic salinity at the entrance of the bay averages 35.3 psu, while the system salinity averages 35.9 psu. Since the system is net evaporative, residual water flow totalling about 152 x 10 ³ m ³ day ^-1 is into the bay. Figure 1 summarizes the water and salt budgets. According to this budget, the water exchange time is about 500 days.

Figure 1. Water and salt budgets for Bahía Concepción, winter. 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 this system The mixing outflow of DIP from this system is substantially larger than the residual inflow and demonstrates that there must be DIP production (DDIP) of approximately +2,637 mol day ^-1 in the system. We assume that this represents decomposition of organic matter. Because of the long water residence time, we assume that this decomposition rate is averaged over a period longer than one year.

N Balance

Similarly, the system shows strong net export of DIN (+10,371 mol day ^-1 ; Figure 2). Again, this outward mixing represents DIN production (DDIN) and is assumed to represent decomposition of organic matter.

Stoichiometric Calculations of Aspects of Net System Metabolism

The rates of DIP and DIN production can be used to estimate the apparent rate of nitrogen fixation minus denitrification (nfix-denit) as the difference between observed and expected DIN production (DDIN_obs-DDIN_exp), where DDIN_exp is DDIP multiplied by the N:P ratio of the reacting particulate organic matter. We assume that this reaction ratio is the Redfield N:P ratio of 16:1, for plankton. Thus:

(nfix-denit) = DDIN_obs-DDIN_obs = DDIN_obs – (N:P)_part x DDIP

(nfix-denit) = +10,276 – 16 x (+2,637) = -31,916 mol day ^-1

Averaged over the area of the bay, (nfix-denit) equals -0.1 mmol N m ^-2 day ^-1 . This system appears to be denitrifying at a relatively slow rate, although the nonconservative flux signal for DIN integrated over longer than a year is a strong signal. In a similar fashion, DDIP multiplied by the negative of the C:P ratio of the reacting organic matter can be used to estimate net ecosystem metabolism (NEM), or production minus respiration (p-r). The reacting organic matter is assumed to have a C:P ratio equal to the Redfield C:P ratio of 106:1:

NEM = [p-r] = [C:P]_part x DDIP

NEM = -106 x 2,637 = -279,522 mol day ^-1 .

Over the bay area, NEM equals –1 mmol m ^-2 day ^-1 ; that is, the system is slightly net heterotrophic. These data are also summarised in the regional comparison tables.

Figure 2. DIP and DIN budgets for Bahía Concepción, winter. Fluxes in mol day ^-11

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Last Updated 14 Jan 2000 by DPS

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