1. Water Depth and Nutrient Ratios over Space
Assessing Drivers of Spatial and Temporal Trends of Nutrient Ratios in the Southern Everglades
Andres Sola E.E. Gaiser, and L. Marazzi - Department of Biological Sciences, Florida International University, Miami FL, USA
Water Depth and Nutrient Ratios over Space
Introduction
Periphyton mats – formed by algae, bacteria, fungi, and small consumers – are complex systems of critical importance for the biogeochemistry of the Everglades and for its resilience to agricultural fertilization, water management, and sea level rise. We investigated the effects of nutrient sources and water depth on carbon (C), and nutrient concentrations, and nitrogen-phosphorus ratios (N:P) in periphyton in Shark River Slough (SRS) and Taylor Slough / Panhandle (TSPh) (Fig 1). Our hypotheses are: 1) periphyton mats at near-canal sites have higher C and nutrient concentrations from agricultural sources due to the proximity to canals; 2) in SRS, sites downstream have lower N:P than upstream sites due to natural P input from the Gulf of Mexico, but not in TSPh, as Florida Bay P inputs are lower; and 3) water level fluctuations control inter-annual variability of nutrient concentrations. High intra-site and inter-annual variance encouraged testing these hypotheses (H1, H2, H3).
Periphytometer at TSPh 2
High TP non-cohesive periphyton mats
Figure 2 – Water level across sites at dates when samples were collected.
Low TP periphyton cohesive mats
ISCO water sampler in SRS 2
Figure 3 – Ratio (in moles) of nitrogen to phosphorus across sites.
Water Depth and Nutrient Concentrations Over Time
Taylor Slough ( )
Shark River Slough ( )
Figure 1 – Everglades National Park and locations of SRS and TSPh sites.
Methods
In , water depth was measured daily and periphyton samples collected three times a year (in January, May and September) from periphytometers – artificial substrata that algae attach to.
Concentrations of total phosphorus (TP), nitrogen (TN), and carbon (TC) were measured in periphyton and N:P molar ratios calculated
Spatial (within and across sloughs) and temporal trends were described and assessed by means of boxplots and multi-variable histograms
Results
Water depth increased from upstream to downstream in SRS, and was overall lower in TSPh (Fig. 2). Mean TP decreased from upstream to downstream in TSPh (Fig. 4) (H1)
In SRS, N:P & C:P ratios (not shown), decreased downstream, except for early years as the upstream site locations were closer to canals; whereas it increased in TSPh, supporting H2 (Fig. 3)
TN, TP, and TC concentrations showed similar temporal trends, particularly in SRS, where TP increased after the dry season (Fig. 4); despite broadly similar water level fluctuations, TN and TP vary more in SRS than in TSPh over time, thus H3 may be supported only in SRS (Figs. 3-4)
Conclusions and Outlook
This analysis of long-term data suggests that the TSPh drainage is more highly influenced by upstream supplies of P (H1) while SRS is controlled by downstream, potentially marine, supplies (H2), confirming the “upside-down” estuary hypothesis proposed by Childers et al. (2006). The nutrient ratios in the TSPh upstream sites may be due to legacy P mobilized through the eastern boundary detention basins (Gaiser et al., 2014). Further analysis is required to assess quantitatively how dry-wet season water level changes control nutrients (H3). Our results are helping us interpret the relative influence of freshwater and marine water sources, and of water level fluctuations on nutrient ratios in periphyton in the Everglades. This preliminary study provides a baseline to predict how nutrient availability to algae and their consumers would change with lower inland nutrient pollution following the desired freshwater restoration outcomes, and with ongoing sea level rise and salt water intrusion.
References:
Childers D.L., Boyer J.N., Davis S.E., Madden C.J., Rudnick D.T., & Sklar F.H Limnology and Oceanography 51:
Gaiser E.E., Sullivan P., Tobias F.A.C., Bramburger A.J., and Trexler J.C Wetlands 34: S55-S64.
Acknowledgements: We thank the FCE LTER program for funding, data and images and Dr. John Kominoski for providing useful comments.
Figure 4 – Water depth, total phosphorus (TP), nitrogen (TN), and carbon (TC) concentrations by mass (dry season months indicated by brackets).