Stable isotopes of oxygen, carbon and nitrogen were used to understand the ecological changes induced by human activities. The existing method developed by McLaughlin et al. (2004) for determination of the oxygen isotopic composition of dissolved phosphate (DIP) is not directly applicable to organic-rich waters in the Florida Everglades and produced Ag3PO4 precipitates contaminated by high amounts of organic matter. A series of laboratory tests was conducted to reduce the amount of organic contaminant in Ag3PO4 prepared from organic-rich DIP samples and to improve the accuracy of oxygen isotopic analysis of DIP. The improved method reduces the organic contamination in the final Ag3PO4 precipitates by more than 10 fold (from >20% C to ~2% C) compared to the previous method and can be used to accurately determine the oxygen isotopic composition of DIP in organic rich aquatic ecosystems if the oxygen isotopic difference between the DIP and dissolved organic matter is less than 13‰.
The improved method was used to examine the oxygen isotopic systematics of DIP and to identify P sources and recycling in the Everglades. Our data reveal seasonal variations in the &delta18O of DIP, with lower values in the summer and higher values in the winter. Our results show that the &delta18O values of DIP collected from the Everglades National Park are in equilibrium with environmental water. This indicates that phosphorus (P) is quickly recycled in areas with low P concentrations. However, most DIP samples collected from areas impacted by high P loading are not in oxygen isotopic equilibrium with water, suggesting that biological cycling of P is not fast enough to erase the “fertilizer” &delta18O signature in the DIP pool in areas with high P levels. Using a two-endmember (i.e., fertilizer P and biologically recycled P) mixing model, we estimated that less than 57.5% of the DIP was derived from fertilizer. The &delta18O of total P (&delta18OTP) in sediments suggests at least three P resources: organic P with depleted 18O, fertilizer P with &delta18O values of ~23.7‰, and bacteria-processed P with &delta18O of ~16 to 24‰ at 20 to 30°C. Our data also show that the sediment P is dominated by organic P. The &delta18O of DIP displayed significant negative correlations with precipitation and evapotraspiration, underscoring the importance of hydrological factors in controlling the P cycle.
Stable isotopes of carbon and nitrogen were used to trace the source of carbon and nutrients and to investigate the food web structure in Missouri River. Our data show that all the fish depend on the same carbon source and there exists significant dietary overlap between the invasive Asian carps and the native filter-feeding fishes. The similarity of &delta13C values of particulate organic matter and the fish muscle tissue indicate that the food base of the filter-feeding fishes was particulate organic matter in the water column. Bighead carp, the hybrids, gizzard shad, silver carp and bigmouth buffalo have trophic positions of 2.9, 2.9, 3, 2.1 and 2.6, respectively. Bighead carp and the hybrids compete with paddlefish for zooplankton and silver carp compete with gizzard shad for algae and detritus. Understanding the competitions between the invaders and native species can help policy makers and ecosystem managers to better manage the aquatic system.