Urbanization plays a minor role in the flooding and surface water chemistry of Puerto Rico’s mangroves

Abstract

Although hydrology and water chemistry are known to change in proximity to cities, there remains little empirical evidence connecting specific components of urban landscapes to mangrove flooding dynamics or surface water chemistry. This study constructs five-year water level models from tidal harmonics and precipitation inputs to characterize mangrove flooding across urban gradients in three watersheds of Puerto Rico. There was some evidence for an influence of urbanization on both flooding and water chemistry, but this depended on the definition of urbanness, and points instead to geomorphology as the primary culprit. Urban sites exhibited 46% longer hydroperiods and 450% lower depths than non-urban sites. Rainfall importance was explained more by geomorphology than by urbanization and suggested systems with limited tidal connectivity are four times more sensitive to rainfall than systems with full tidal connectivity. There was also evidence for changes in tidal amplitudes along the urban gradient, which may explain the observed differences in flooding. Relationships between surface water chemical metrics and land cover contradicted previous studies by suggesting lower nutrients and biochemical oxygen demand with increasing urbanization. These results reinforce the understanding that the most important drivers of urban mangrove hydrology and water quality in Puerto Rico are likely geomorphology and tidal connectivity, with little but not zero influence from surrounding land cover. Results should be considered alongside the reported errors stemming from inaccuracies in digital elevation and rainfall response models, and will be useful in understanding future ecological censuses on the island.

Mangrove leaf and root traits and their relation to urbanization

“…It's likely this relationship is not directly tied to the urbanization variables because point-source sewage inputs only influence the nearest mangrove sites. This was evident in the heterogenous surface water nitrogen and phosphorus concentrations from (Branoff 2018b).…”

“…Isotope values are thus expressed in reference to these standards as δ notation where δX (‰) = [(Rsample / Rstandard) -1] × 103, and X is 13 C or 15 N and R is 13 C/ 12 C or 15 N/ 14 N isotopic ratio, respectively. Carbon isotope ratios from herbaria were further corrected for global atmospheric changes in 13 C/ 12 C with time as a result of anthropogenic combustion of fossil fuels (Suess 1955;Feng 1998 Surrounding land cover metrics at each site were extracted from various spatial datasets and used to characterize the urbanization of each site within a 0.5 km radius as described in more detail in (Branoff 2018b surfaces within the sampling area. These variables were used individually as predictors for measured morphology and chemistry traits, as was an urban index that includes them all and is taken as a relative level of urbanization between the sites.…”

“…Surface water chemistry measurements were taken from the San Juan Bay Estuary Program, as described in (Branoff 2018b), and are only available for San Juan sites (Figure 1).…”

“…Biannual metrics were total Kjeldahl nitrogen (mg/L), nitrate and nitrite total (mg/L), ammonium (mg/L), total phosphorous (mg/L), and fecal and enterococcus coliform bacterial counts (CFU/100mL). Flooding metrics were aggregated at the site level from water level models also representing the period of 2012 to 2017, and which were constructed from water-level observations at piezometers throughout the mangroves as described in (Branoff 2018b) (Figure 1). Flooding metrics include the proportion of time flooded (%), mean flood length (h), mean flood frequency(day -1 ), and mean flood depth (m).…”

Urbanization drives the acoustic diversity of mangrove anurans and avifauna in Puerto Rico

“…The urban mangroves of Puerto Rico, and the sites corresponding to the present study, are described in detail in Branoff and Martinuzzi (2018). Surface water chemistry and flooding dynamics are described in (Branoff, 2018). Twenty one-hectare forested areas were selected among three watersheds, which were chosen because they harbored mangroves with the greatest range in urbanization (Figure 1).…”