Potential changes in bacterial metabolism associated with increased water temperature and nutrient inputs in tropical humic lagoons

Abstract

Temperature and nutrient concentrations regulate aquatic bacterial metabolism. However, few studies have focused on the effect of the interaction between these factors on bacterial processes, and none have been performed in tropical aquatic ecosystems. We analyzed the main and interactive effects of changes in water temperature and N and P concentrations on bacterioplankton production (BP), bacterioplankton respiration (BR) and bacterial growth efficiency (BGE) in tropical coastal lagoons. We used a factorial design with three levels of water temperature (25, 30, and 35°C) and four levels of N and/or P additions (Control, N, P, and NP additions) in five tropical humic lagoons. When data for all lagoons were pooled together, a weak interaction was observed between the increase in water temperature and the addition of nutrients. Water temperature alone had the greatest impact on bacterial metabolism by increasing BR, decreasing BP, and decreasing BGE. An increase of 1°C lead to an increase of ~4% in BR, a decrease of ~0.9% in BP, and a decrease of ~4% in BGE. When data were analyzed separately, lagoons responded differently to nutrient additions depending on Dissolved Organic Carbon (DOC) concentration. Lagoons with lowest DOC concentrations showed the strongest responses to nutrient additions: BP increased in response to N, P, and their interaction, BR increased in response to N and the interaction between N and P, and BGE was negatively affected, mainly by the interaction between N and P additions. Lagoons with the highest DOC concentrations showed almost no significant relationship with nutrient additions. Taken together, these results show that different environmental drivers impact bacterial processes at different scales. Changes of bacterial metabolism related to the increase of water temperature are consistent between lagoons, therefore their consequences can be predicted at a regional scale, while the effect of nutrient inputs is specific to different lagoons but seems to be related to the DOC concentration.

Keywords: bacterial growth efficiency; bacterial metabolism; coastal lagoons; environmental changes; tropical aquatic ecosystems; water temperature.

FIGURE 1

Changes in bacterial respiration, bacterial production, and bacterial growth efficiency (BGE) in relation to the increase in water temperature in five humic tropical lagoons (Cabiunas – Cab, Carapebus – Carap, Comprida – Comp, Amarra-Boi – Aboi, and Atoleiro – Atol). For each temperature, all bacterial data related to different nutrient additions were pooled together because temperature effects were consistent for all lagoons and independent of nutrient additions (see Table 2 for the analysis results). Colored thin lines link the medians of bacterial data for each lagoon at each incubation temperature. The black central line represents the best fitting line that predicts bacterial respiration (BR), production (BP), or growth efficiency (BGE) in response to water temperature. Intercepts and slopes of best fitting lines for BR, BP, or BGE are: -0.055 and 0.0144 (BR), 0.128, and -0.0009 (BP), and 0.488, slope = -0.0116 (BGE).

FIGURE 2

Bacterial respiration (upper), bacterial production (middle), and BGE, lower) in relation to different nutrient additions in five humic tropical lagoons. Lagoons were grouped in different panels based on the bacterial responses to nutrient additions. Panels on the left show data from the lagoons with lower carbon concentrations (Cabiunas – Cab, Carapebus – Carap) while panels on the right show data from the lagoons with higher carbon concentrations (Comprida, Comp, Amarra-Boi – Aboi, Atoleiro – Atol). Control – no nutrient addtion, N – nitrogen addition, P – phosphorus addition, and NP – nitrogen and phosphorus additions. Asterisks indicates a significant difference in response of bacterial metabolism to nutrient additions compared with the control for each lagoon (*p < 0.05, **p < 0.01, ***p < 0.001). Note that no comparison between lagoons is showed here.