Submarine groundwater discharge and nutrient enrichment in Kingston Harbour, Jamaica: geochemical evidence and spatial and seasonal dynamics

Submarine groundwater discharge (SGD) constitutes a critical but systematically underquantified pathway for nutrient delivery to coastal waters, particularly in Small Island Developing States (SIDS) where coastal aquifers may be contaminated by wastewater and urban runoff. This study evaluated spatial and temporal variability in nutrients, chlorophyll a and physicochemical conditions in Kingston Harbour, Jamaica, a coastal water body that has experienced decades of eutrophication and declining biodiversity. This investigation targeted subsurface, low-salinity anomalies consistent with potential fresh SGD (FSGD) which had been identified as a source of land-based pollution in previous studies. Between May 2022 and August 2023, conductivity-temperature-depth (CTD) profiling was conducted to examine potential FSGD sites, along with surface water and targeted-depth water sampling at five northern shore sites which showed recurrent subsurface salinity anomalies. Station blocked permutation tests showed significant temporal variability in all measured surface and targeted depth parameters after Holm adjustment, while date-blocked analyses indicated significant spatial variability in surface waters but not at depth. Paired site-date comparisons did not show systematic enrichment of total oxidized nitrogen (TOxN), total dissolved nitrogen (TDN), soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP), soluble reactive silicate (SRSi), chlorophyll a or salinity at targeted depths relative to corresponding surface waters, but depth waters differed significantly from paired surface waters in pH and temperature. Antecedent rainfall was analyzed at the sampling-event level at 1, 3, 5, 7 and 14-day intervals to assess rainfall impacts with the strongest associations observed for 3-day antecedent rainfall with salinity and SRSi. Surface salinity was also significantly associated with multiple nutrient and chlorophyll a variables, including an inverse SRSi-salinity relationship. At potential FSGD sites, TDN was strongly positively associated with salinity at depth. A monotonic SRSi-salinity relationship was not significant after adjustment, but a low-complexity generalized additive model supported a nonlinear depth SRSi-salinity relationship. The results provide geochemical evidence consistent with localized freshwater influence from potential FSGD. Direct tracer-based flux measurements and groundwater endmember characterization are needed to resolve SGD contributions to nutrient loading in this harbor.