David Alejandro Sánchez-Núñez

The temporal variability of fish habitat utilization is poorly understood in tropical deltaic systems due to high water turbidity, which limits visual censuses, and to the lack of long-term data incorporating climate variability events. We aimed to assess the influence of body size and El Niño Southern Oscillation (ENSO) variability on the cross-habitat utilization rate of 14 fish species of commercial relevance in the Ciénaga Grande de Santa Marta (CGSM). We estimated the utilization of mangroves and coastal lagoons based on relative catch frequencies from encircling gillnets used within a long-term catch monitoring program, and then tested for significant changes in each species’ habitat utilization as a function of body size and climate variability. Six species showed a high dependence on mangroves and four on coastal lagoons for most body size classes (including juveniles) and ENSO conditions. One species (Elops smithi) showed a high utilization of mangroves in some ENSO phases and body size classes, while three species showed a high utilization of both mangroves and coastal lagoons. Mangrove utilization by six species (Megalops atlanticus, E. smithi, Centropomus undecimalis, Mugil incilis, Mugil liza, and Ariopsis canteri) increased in larger body sizes at low depths, which usually occurs under dry ENSO conditions, when predatory risk is higher in coastal lagoons. Another species (Caquetaia kraussi) increased its mangrove utilization from the body size at which its feeding habits change. Mangroves and coastal lagoons are important nurseries and habitats for adults of the main commercial fish species in the CGSM. Seascape habitats and fringe/riverine mangroves must be conserved in tropical deltas to promote not only nurseries but also fish lifecycles.

Hydrological rehabilitation is widely used to improve soil conditions and promote passive recovery. However, climate variability that is seldom considered in restoration planning can affect hydrological rehabilitation goals. We used long‐term observations to assess the effects of El Niño Southern Oscillation (ENSO) climate variability and hydrologic rehabilitation on tree recruitment and landscape cover changes in a South American mangrove complex. ENSO climate variability and rehabilitation hydrology measure types collectively explained 88%, 90% and 70% of mangrove cover change rates in the whole mangrove system and porewater salinity in basin and riverine sites, respectively. Therefore, major and detrimental rehabilitation measures counterbalanced or reinforced the effects of ENSO phase intensity on the system. In addition, climate variability explained the rates of change of salinity in the basin site (58%). Porewater salinity, its rates of change and ENSO explained 28%–75% of propagule density and seedling/sapling growth rates ofRhizophora mangle,Laguncularia racemosaandAvicennia germinansin representative basin and riverine sites. Tree colonization in the basin site, under high irradiance, was triggered by high seedling/saplings diameter growth rates (0.5–1.6 cm year⁻¹) ofL. racemosaandR. manglegenerated by strong porewater salinity drop rates during consecutive La Niña episodes, while widespread tree mortality occurred during a strong El Niño. Tree size categories were more stable to ENSO climate variability in the riverine site. Synthesis and implications. This study shows that hydrologic rehabilitation effectiveness depended on its influence on freshwater flows and on the effects of ENSO climate variability. ENSO, porewater salinity and its rates of change regulated tree recruitment and landscape cover changes. Thus, besides porewater salinity, rates of change of porewater salinity, light availability and climate variability need to be included in monitoring and mangrove restoration planning to better understand and predict mangrove trajectories. Climate‐smart restoration in mangroves should implement the types of hydrological rehabilitation measures that offset or avoid reinforcing ENSO strong phases.

Mangroves provide multiple ecosystem services and are essential for mitigating global warming owing to their capacity to store large carbon (C) stocks. Due to widespread mangrove degradation, actions have been implemented to restore them worldwide. An important representative case in Colombia is the Ciénaga Grande de Santa Marta’s restoration plan. This management intervention focused on restoring the natural hydrological functioning after massive mangrove mortality (~25,000 ha) due to soil hyper-salinization after river water input from the Magdalena River was eliminated. A partial recovery occurred during subsequent years, and hydrological management is still being implemented today. To understand how the degradation and subsequent management have affected mangrove C stocks, we compared C stocks in stands with different intervention levels reflected in their current forest structure. We found that the total C stock (398–1160 Mg C ha⁻¹) was within the range measured in other neotropical mangroves without vegetation deterioration. The aboveground C was significantly higher in the stands where hydraulic connectivity was restored. By contrast, the belowground C was higher in the stands with low hydraulic connectivity due to channel clogging and a lack of sufficient maintenance. Our results show that hydrological management measures influenced above- and belowground C stocks, even at a 2 m depth. In addition, a strong indirect relationship useful for estimating carbon content from organic matter content was found.