Recent Sedimentary History of Organic Matter and Nutrient Accumulation in the Ohuira Lagoon, Northwestern Mexico
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- Ruiz-Fernández, A.C., Frignani, M., Tesi, T. et al. Arch Environ Contam Toxicol (2007) 53: 159. doi:10.1007/s00244-006-0122-3
210Pb-derived sediment accumulation rates, as well as a suite of geochemical proxies (Al, Fe, δ13C, δ15N), were used to assess the time-dependent variations of C, N, and P fluxes recorded in two sediment cores collected at Ohuira Lagoon, in the Gulf of California, Mexico, during the last 100 years. Sedimentary C, N, and P concentrations increased with time and were related to land clearing, water impoundment, and agriculture practices, such as fertilization. C:N:P ratios and δ13C suggested an estuarine system that is responsive to increased C loading from a N-limited phytoplankton community, whereas δ15N values showed the transition between an estuarine-terrestrial to an estuarine-more marine environment, as a consequence of the declining freshwater supply into the estuary due to the channeling and impoundment of El Fuerte River between 1900 and 1956. The recent increases in nutrient fluxes (2- to 9-fold the pre-anthropogenic fluxes of C and N, and 2 to 13 times for P) taking place in the mainland from the 1940s, were related to the expansion of the intensive agriculture fields and to the more recent development of shrimp farming activities.
The Mexican state of Sinaloa has 656 km of coastline and is characterized by high fishing and agriculture activity. Ohuira is a shallow and brackish coastal lagoon (average depth of 3 m, surface area of 125 km2), which supports local fishing activity including wild and cultivated shrimp for export. However, there is some concern related to the desiccation of lowlands and infilling promoted by sediment transport due to agriculture activities, as well as alteration of water quality due to waste water discharges (CONABIO 2002). Actually the lagoon receives the untreated domestic wastes from the village of Juan José Ríos (23,470 inhabitants; INEGI 2000) as well as the residual waters from 4 shrimp aquaculture facilities (350 ha; Lyle-Fritch et al. 2001) and El Fuerte Valley agriculture lands, which are characterized by intensive tillage, irrigation, heavy application of fertilizers and pesticides, and monocropping.
The economic development of the region started about 1890 upon the efforts of a group of utopian socialists from the United States, who built the first irrigation system in El Fuerte Valley to develop the sugar cane industry (Gill 2003). Currently El Fuerte Valley represents approximately 15% of the irrigated lands in Mexico, and its most important crops are sorghum, corn, sugar cane, mangos, alfalfa, and chick peas, with 26, 12, 19, 2, 2, and 1 × 103 cultivated hectares, respectively (SAGARPA 2005).
The aim of this work was to evaluate the temporal changes in sediment accumulation rates and excess nutrient fluxes reflected in the sedimentary record of C, N, and P in the Ohuira Lagoon mudflats, using 210Pb chronology. C and N isotope composition was used to evaluate the relationship between the observed changes and the economic development of El Fuerte Valley.
Materials and Methods
Water content data were used to calculate the porosity and in situ dry sediment bulk density, to estimate the mass depth (accumulated weight per area unit, g cm−2) for each core section (Baskaran and Naidu 1995). Organic matter (OM) and carbonate (CaCO3) contents were estimated through loss on ignition (LOI) at 400°C and 750°C, respectively; OM was calculated as 0.58*LOI400 (EPA 2006). Total phosphorus (P) was measured as molybdate reactive P by UV spectrometry, after calcination of the sediment samples (550°C, 1 h; Páez-Osuna et al. 1991) with an analytical precision of 4.8%. Organic carbon (OC) and total nitrogen (N) contents were measured using a FISONS NA2000 Element Analyzer after removal of the carbonate fraction in silver capsules using 1.5 M HCl. The average standard deviation of each measurement was determined by replicate analyses of the IAEA standard NBS19 (0.07 % for OC and 0.009 % for N).
Stable isotopic analyses of OM (13C/12C and 15N/14N) were carried out on the same samples using a Finnigan Delta Plus mass spectrometer, which was directly coupled to a Fisons NA2000 EA by means of a CONFLO interface for continuous flow measurements. Stable nitrogen and carbon isotope ratios are reported in the conventional δ-notation with respect to atmospheric N2 (air) and PDB (Pee Dee Belemnite) carbonate standard, respectively. Uncertainties were lower than ± 0.2‰, as determined from routine replicate measurements of the reference sample standard IAEA-NBS19 for the δ13C and standard IAEA-N-1 for the δ15N.
Sediments for Al, Fe, and 210Pb analyses were digested overnight in closed SavillexTM containers, on a hot plate at 200°C, in a mixture of 5:4:1 HNO3 + HCl + HF (Loring and Rantala 1992). Al and Fe were analyzed by AAS (uncertainties of 2–5%, determined from replicated analysis of the standards IAEA-356 and BCS-1). Total 210Pb (210Pbtot) was determined by alpha counting of 210Po spontaneously deposited onto silver discs (Flynn 1968) using 209Po as yield tracer. Uncertainty (± 4.6%) was determined through replicate measurements of standard IAEA-300. 137Cs and supported 210Pb (210Pbsup, derived from the 214Bi photopeak) were measured on Ortec gamma detectors for a minimum of 24 h. Excess 210Pb (210Pbxs) activities were determined by subtracting 210Pbsup from 210Pbtot values.
Results and Discussion
OM/Al ratios exhibited an increasing trend from around 11 cm depth to the surface at the 2 sites (Figure 2E) suggesting an increment in the biogenic contribution over the lithogenic supply to the site, although OM diagenesis could have also influenced the OM/Al ratios decreasing with depth. Nonetheless, this trend also confirmed that these sediments are not redeposited sediments since, otherwise, the sediment profile would have exhibited uniform OM/Al ratio distribution (Emeis et al. 2000).
Short-Lived Radioisotopes (137Cs, 210Pb)
137Cs activities in both cores were below the analytical background, as has been previously observed in other sediments from the Mexican Pacific coast (Páez-Osuna and Mandelli, 1985; Ruiz-Fernández et al. 2001, 2002) and explained as a consequence of poor fallout of this radionuclide over these low latitude coastal areas.
Figure 3A and B shows that both logarithmic 210Pb activity depth profiles are characterized by oscillations that account for changes of accumulation rates over time. However, in the case of core OH2 an average mass accumulation rate (g cm−2 y−1) could still be calculated through the Constant Flux-Constant Sedimentation model (CF-CS; Appleby and Olfield 1992). Differently, the OHT 210Pbxs profile showed two different main trends (from surface to 6 cm depth, and from 6 to 16 cm depth) suggesting a recent abrupt change of accumulation rate. In this case, the 210Pb dating was calculated using the Constant Initial Concentration model (CIC; Appleby and Oldfield 1992) that allows dating non-linear 210Pbxs profiles. 210Pbxs depth profiles allowed dating the sediment cores only down to 12 cm depth for core OHT and to 17 cm depth for OH2, corresponding to a time span of circa 100 years (115 ± 12 and 106 ± 11 years), the most reliable period of time for 210Pb chronologies.
Organic carbon content in both sediment cores showed the typical exponential decay with depth (r > 0.96, p < 0.05). Elemental analyzer-OC values were considerably lower (between 2- and 10- fold) than those estimated from LOI400-derived OM although both profiles showed similar trends.
Concentrations of OC, N, and P in estuarine sediments
P (mg g−1)
Ohuira Lagoon, México
Culiácan River Estuary, México
Ruiz-Fernández et al. (2002)
Chiricahueto Lagoon, México
Soto-Jiménez et al. (2003)
Altata-Ensenada del Pabellón, México
Páez-Osuna et al. (1992)
Celestún Lagoon, México
Gonneea et al. (2004)
Chesapeake Bay Estuary, USA
Cornwell et al. (1996)
Winyah Bay Estuary, USA
Goñi et al. (2003)
Florida Bay Estuary, USA
Orem et al. (1999)
Seine Estuary, France
Mesnage et al. (2002)
Shinji Lagoon, Japan
Cochin Estuary, India
Ashtamudy Estuary, India
Babu et al. (1998)
Mandovi Estuary, India
Nasnolkar et al. (1996)
Lunawa Estuary, Sri Lanka
Ratnayake et al. (2005)
Sedimentation associated with tidal flooding is an important source of both N and P to tidal marshes with N usually linked mainly to organic matter and P primarily associated with the fine grained clay minerals (Morse et al. 2004). OC correlation with P and N likely suggests the importance of this organic-rich deposit as a nutrients reservoir. The correlation between OC and N indicates that the sources that supply OM to these sediments are present in nearly invariant proportions (Ruttenberg and Goñi 1997), while the correlations between OC and P, as well as N and P, which were found only in the superficial layers of both cores (topmost 10 cm) are either indicating a change in phosphorus main incorporation route or are simply the result of OC enrichment, since P and OC concentrations significantly correlate with each other only in high organic coastal sediments (Ingall and van Cappellen 1990).
Organic carbon diagenesis versus nutrient enrichment
Parameters of first-order G model for OC and ON decomposition at Ohuira Lagoon
Residence time (year)
OCmetabolized fraction (%)
Residence time (year)
OCmetabolized fraction (%)
The Middelburg model underestimated most of the OC concentrations at OHT but described relatively well OC changes in core OH2 up to 11.5 cm, indicating that OC content can be accounted for by steady-state diagenesis up to that depth. The OC concentrations were higher than those predicted by the model, reflecting an increase in OC deposition starting from the early 1900s at OHT, and from the early 1940s at OH2.
Geochemical variable ranges obtained for sediment cores from Ohuira Lagoon
OCbck (mg g−1)
0.48 ± 0.02
0.25 ± 0.01
N (mg g−1)
Nbck (mg g−1)
0.80 ± 0.02
0.40 ± 0.02
P (mg g−1)
Pbck (mg g−1)
14.6 ± 0.6
15.2 ± 0.2
−1 to + 10
+7 to + 12
−14 to −18
−14 to −18
210Pbtot (dpm g−1)
Accumulation rate (g cm−2 y−1)
Sedimentation rate (cm y−1)
OC fluxes (mg cm−2 y−1)
N fluxes (mg cm−2 y−1)
P fluxes (mg cm−2 y−1)
Anthropogenic P fluxes (Figure 5C) were obtained by multiplying the excess P concentrations (Pxs) by the 210Pb-derived sediment mass accumulation rates. Pxs were calculated by subtracting the pre-anthropogenic P values (Pbck,) from the P concentrations analytically determined. Pbck are often obtained from the correlation between P and Fe in pristine sediments (Gerritse et al. 1998) under the assumption that Fe-(hydroxy)-oxides tend to adsorb the inorganic P (Shukla et al. 1971); however, none of the sediment cores displayed such a relationship, likely due to their reducing conditions. Pbck values were, therefore, obtained from the average of the asymptotic values observed below 23 and 15 cm depth at OHT and OH2 cores, respectively (Table 3).
As with C and N fluxes, P fluxes were higher at OHT than at OH2. The larger nutrient enrichment observed at site OHT is most likely due to a preferential sedimentary dynamics, since site OH2 is more influenced by tides than OHT, which is currently accreting at higher sedimentation rates and receiving a higher supply of nutrients due to its proximity to the agriculture fields of El Fuerte Valley and some shrimp aquaculture facilities, established since the late 1980s. Considering that the effective sedimentary retention of the total amount of nutrients supplied to the estuaries can be as low as 40% for N and usually less than 60% for P (Tappin 2002), the N and P fluxes calculated for Ohuira cores might underrate the historical nutrient supply to the lagoon.
Organic matter provenance
The C:P and N:P values were rather low since the pre-anthropogenic times (pre-1900), suggesting a prevailing phosphorus clastic source in the area. Their increasing trends, based on the significant correlation found between OC and the C:P and N:P ratios (p < 0.05, r2 > 0.95) were associated with the increased anthropogenic input of OM.
Sedimentary OM found in Ohuira cores fits in the reported ranges of δ15N (−1.2 to +10.6‰; Cloern et al. 2002) and C:N (6 to 9; Giordani and Angliolini, 1983) for coastal and estuarine-marine organic matter, although δ13C values were higher than expected for estuarine-marine OM (−17 to −28‰; Cloern et al. 2002).
The decreasing δ13C values are usually related to the decrement of algal productivity with the enhancement of the isotopic discrimination against dissolved 13CO2 (Tenzer et al. 1999). However, the C:N ratios in both cores showed that the marine algal contribution did not decrease with time and, therefore, the declining δ13C values could result from the contribution of more marine OM (−20 to −22‰; Meyers 1994).
According to Meyers (1997), the observed δ15N profile could be the result of the reduction of freshwater supply after the partial diversion of a river. According to the plot of δ15N and δ13C (Figure 7A), the sediment core OHT is showing the transition between two kinds of environments: (1) from the bottom of the core up to 11.5 cm depth (year 1900), a wetter environment in which estuarine OM has a more terrestrial character; and (2) from 9.5 cm depth (early 1930s) to the surface, a dryer environment in which estuarine OM has a more marine character (the values from sediment core OH2 are also located in this region of the plot). δ15N and δ13C values increasing together can be produced by dryer conditions (runoff diminution) with bloom maxima migrating upstream (as the saline plume arrives further up in the estuarine zone) and decreased size of the bloom due to lower nutrient flux to the site (Bratton et al. 2003). The 15N increase is likely due to the decreasing input of isotopically light land-plant detritus carried by the river waters to the estuary (Meyers and Lallier-Vergés 1999) where marine algae are more abundant.
C and N isotopic changes are undoubtedly related to the development of the agroindustry in the surrounding area of Ohuira Lagoon. The period of gradual decreasing δ13C values in core OHT corresponded with the gradual increment of the cultivated area at El Fuerte Valley (Figure 6F) between 1900 and 1959, whereas in core OH2 the evidence is observed after the abrupt increase of cultivated areas between 1956 and 1957. On the other hand, δ15N increasing values are most likely the result of the reduction in fresh water supply to the site due to the creation of irrigation infrastructure of El Fuerte Valley and the consequent development of more brackish-marine conditions. Actually, the Tastes channel was built in 1892 to derive water from the El Fuerte River to the Los Mochis desert. By the mid-1940s, this channel and several other minor channels connected through 500 pumps were used to irrigate 18,000 ha. The SICAE channel was built by 1947 and the irrigated area abruptly doubled from 18,000 to 36,000 ha; and with the construction of Miguel Hidalgo dam in 1956, the irrigation capacity increased from 55,000 to 240,000 ha (Gill 2003).
The combination of geochemical analysis and 210Pb dating was very useful to examine the past rates of sediment and nutrient sequestration in a coastal environment. C and N isotope ratios proved to be more sensible tools to trace changes in OM provenance than C:N:P ratios. The study demonstrated that anthropogenic land use within the catchment is the main factor driving both sediment and nutrient retention in two mudflat areas of Ohuira Lagoon; although the impact is not spatially uniform and, therefore, care should be taken to avoid generalizations. The different effects of the nutrient-enriched runoff from agricultural lands observed in the two cores were related to the hydrological regime (open and partially closed subtidal areas) and the closeness to the sediment and nutrients source. Sediment accumulation in the area closer to the culture fields has increased ∼5 times with respect to the pre-anthropogenic conditions in less than 20 years, while C, N, and P fluxes increased almost 10-, 10-, and 13-fold, respectively. At the site that is more influenced by marine conditions, no changes in sediment accumulation were observed, although C, N, and P fluxes doubled during the last 60 years.
Financial support was partially provided by projects CONACyT-SEMARNAT-2002-C01-0161, DGAPA IN103605-3, and Mobility Actions CONACyT-CNR Italy 2006–2008; the travel grants to A.C.R.F. were provided by ISMAR-CNR and the Program UNAM-CIC of International Academic Exchange. Thanks are due to G. Ramírez-Reséndiz, M.C. Ramírez-Jáuregui, and V. Montes-Montes (ICMyL, UNAM) for their technical assistance and to S. Regan for English revision. A.C.R.F. is also grateful to S. Romano, S. Albertazzi, and S. Giuliani for their constant support during her stay at ISMAR-CNR, Bologna, Italy. This is contribution No. 1527 from CNR-ISMAR, Bologna.