-This study examines the evolution of Lough Furnace, a coastal brackish lake in the west 11 of Ireland, using high-resolution sensors in the water column and palaeolimnological examination 12 of the sediment archive. Palaeoenvironmental reconstructions suggest that meromixis formed as a 13 result of sea level rise prior to ca. 4,000 cal. yr BP. Increased seawater inflow has progressively led 14 to permanent water stratification, which caused the onset of anoxia, making the monimolimnion a 15 harsh environment for biological life. Diatom floristic interpretations suggest a progressive upcore 16 increase in salinity, which is paralleled by a reduction in cladocera remains. Diagenetic processes 17have not altered the sediment organic matter signature. Organic matter mainly derives from 18freshwater DOC and appears to be linked to the presence of peat bogs in the catchment as 19 confirmed by the C/N ratio. Upcore variations in the C/N ratio with a ca. 800-year periodicity have 20 been interpreted as the result of alternating dry and wet climatic phases during the late Holocene, 21 which appear synchronous with the NAO and long-term solar cycles. The current hydrology is 22 largely controlled by freshwater inflow, which determines permanent meromictic conditions. 23Overturns are rare, requiring a specific combination of factors such as exceptionally dry and warm 24 summers followed by cool autumns. According to the climate projections for the next century in 25Ireland, permanent meromictic conditions will probably continue. 26 2 27

The mobilisation of energy from allocthonous carbon by heterotrophic bacterioplankton can be proportionally more important than autotrophic production in humic lakes. Moreover, increasing levels of dissolved organic carbon (DOC) in many aquatic systems linked to increases in precipitation, which in turn may be related to changing climate, mean that this heterotrophic component of the food web may play an increasing role in the overall transfer and production of energy, particularly within peatland catchments. While such catchments are common in the temperate northwest Atlantic regions of Europe, studies describing the seasonal dynamics of the heterotrophic, mixotrophic, and autotrophic components of their aquatic food webs are rare. In this study, the biomass of these pelagic components was enumerated over 1 year in 2 oligotrophic lakes, both situated in peatland catchments in the west of Ireland but with contrasting DOC concentrations. Bacterial biomass dominated the pelagic food web of the more humic lake, Lough Feeagh, while autotrophic phytoplankton biomass was greatest in the clearwater lake, Lough Guitane. The biomass of potentially mixotrophic flagellates was also slightly larger in the Lough Guitane, while phagotrophic ciliate biomass was comparable between the 2 lakes. An extreme precipitation event led to a significant increase in bacterial biomass while simultaneously depressing autotrophic production for several months in the humic lake. Extreme precipitation in the clearwater lake also depressed autotrophic production but did not give rise to significant increases in bacterial biomass. This quantification of autotrophic, mixotrophic, and heterotrophic components provides a vital first step in understanding how pelagic communities contribute to net ecosystem productivity, and thus how Irish peatland lakes may be affected by projected climate changes.

Lough Murree, a rock/karst barrier lagoon, is superficially isolated from the sea and seasonal variations in lake water level reflect precipitation and groundwater variation. Lake salinity is influenced by subsurface saline intrusions, occasional barrier overwash together with precipitation and groundwater inflow, leading to poikilohaline conditions. Palaeolimnological reconstructions in Murree support the supposition that the lagoon was once superficially connected to the sea around the mid-nineteenth century. Physical, chemical and biological proxies suggest an evolution to more freshwater conditions. Uncertainties about the timing of the transition persist because of an unresolved sediment chronology. The isolation of Murree from the Atlantic Ocean has promoted the formation of dense charophyte beds composed of lagoonal specialist species, which are able to tolerate large variations in salinity.

Lough Murree, a rock/karst barrier lagoon, is superficially isolated from the sea and seasonal variations in lake water level reflect precipitation and groundwater variation. Lake salinity is influenced by subsurface saline intrusions, occasional barrier overwash together with precipitation and groundwater inflow, leading to poikilohaline conditions. Palaeolimnological reconstructions in Murree support the supposition that the lagoon was once superficially connected to the sea around the mid-nineteenth century. Physical, chemical and biological proxies suggest an evolution to more freshwater conditions. Uncertainties about the timing of the transition persist because of an unresolved sediment chronology. The isolation of Murree from the Atlantic Ocean has promoted the formation of dense charophyte beds composed of lagoonal specialist species, which are able to tolerate large variations in salinity.