Response of a stoichiometrically imbalanced ecosystem to manipulation of nutrient supplies and ratios

Abstract

Cuatro Ciénegas Basin (CCB) is a desert ecosystem that hosts a large diversity of water bodies. Many surface waters in this basin have imbalanced nitrogen (N) to phosphorus (P) stoichiometry (total N:P > 100 by atoms), where P is likely to be a limiting nutrient. To investigate the effects of nutrient stoichiometry on planktonic and sediment ecosystem components and processes, we conducted a replicated in situ mesocosm experiment in Lagunita, a shallow pond located in the southwest region of the basin. Inorganic N and P were periodically added to mesocosms under three different N:P regimes (P only, N:P = 16 and N:P = 75) while the control mesocosms were left unamended. After three weeks of fertilization, more than two thirds of the applied P was immobilized into seston or sediment. The rapid uptake of P significantly decreased biomass C:P and N:P ratios, supporting the hypothesis that Lagunita is P-limited. Meanwhile, simultaneous N and P enrichment significantly enhanced planktonic growth, increasing total planktonic biomass by more than 2-fold compared to the unenriched control. With up to 76% of added N sequestered into the seston, it is suspected that the Lagunita microbial community also experienced strong N-limitation. However, when N and P were applied at N:P = 75, the microbes remained in a P-limitation state as in the untreated control. Two weeks after the last fertilizer application, seston C:P and N:P ratios returned to initial levels but chlorophyll a and seston C concentrations remained elevated. Additionally, no P release from the sediment was observed in the fertilized mesocosms. Overall, this study provides evidence that Lagunita is highly sensitive to nutrient perturbation because the biota is primarily P-limited and experiences a secondary N-limitation despite its high TN:TP ratio. This study serves as a strong basis to justify the need for protection of CCB ecosystems and other low-nutrient microbe-dominated systems from anthropogenic inputs of both N and P.

Fig 1. N:P ratio of residual dissolved nutrients in the water column on days 21 and 42.

Each bar represents average of 5 measurements ± 1 standard deviation. Different letters indicate significant differences between treatments for a given date while ‘*’ represents a significant change from day 21.

Fig 2. Effects of fertilizer application on a) seston C and b) dissolved organic carbon, and their recovery.

Each bar represents average of 5 measurements ± 1 standard deviation. Different letters indicate significant difference between treatments for a given date while ‘*’ represents a significant change from day 21.

Fig 3. Water column chlorophyll a concentrations during fertilizer application (days 6 and 21) and after cessation of fertilizer application (day 42).

Each bar represents average of 5 measurements ± 1 standard deviation. Different letters indicate a significant difference between treatments for a given date. Symbol ‘#’ represents a significant change from day 6 while ‘*’ represents a significant change from day 21.

Fig 4. Effects of fertilizer application on sediment microbe biomass (separated cells) a) C:N, b) C:P, and c) N:P molar ratios for samples collected on day 21.

Each bar represents the average of two measurements (three for NP16 treatment) ± 1 standard deviation.