On ocean beds around the world sinking particles of residual biomass from animals and vegetation have, over time, accumulated into thick layers of sediment. For researchers these layers of sediment contain valuable information about the earth’s past climate, information that can be used to predict future climate change and to estimate natural vs. human causes of today’s global warming.The Institute of Geosciences at Stockholm University studies sediment cores that have been acquired with an apparatus called piston corer. For future coring expeditions additional piston corers are needed. However, the dimensions of the current piston corer only allow use of specially ordered polycarbonate pipes which are used as liners. This pipe is relatively expensive compared to pipes of other plastic materials and of standard dimensions. In order to lower the cost of upcoming coring expeditions a new piston corer design, that allows use of standard dimension plastic pipes as liners, is required.The process of redesigning the piston corer offers an opportunity to identify structural disadvantages and generate ideas on improvements that can be accomplished in a new construction. It also offers an opportunity to satisfy a need to increase the dimensions of the liners in order to acquire a larger volume of sediment, thereby increasing the reliability of the information gathered when examining the sediment. This thesis paper presents the process of redesigning and improving the piston corer currently used by Stockholm University. The resulting design allows use of standard dimension PVC pipes as liners. It has also been improved regarding the process of extruding liners after recovering sediment cores as well as performance and handling.

This thesis investigates how benthic fluxes of phosphorus (P), nitrogen (N), and silicon (Si) change upon oxygenation of anoxic soft bottoms in the brackish, eutrophicated Baltic Sea. Direct measurements in situ by benthic landers demonstrated that fluxes of dissolved inorganic P (DIP) from anoxic bottom sediments in the Eastern Gotland Basin are higher than previously thought (Paper I). It is argued that the benthic DIP flux has a much larger influence on the DIP inventory in the Baltic proper than the external sources. Similarly, benthic fluxes of DIP and dissolved inorganic N (DIN) from anoxic sediment in the coastal Kanholmsfjärden Basin, Stockholm archipelago, were sufficiently high to renew the pools of these nutrients below the upper mixed layer in roughly one year (Paper II). A natural inflow of oxygen rich water into the deep, and previously long-term anoxic part of Kanholmsfjärden Basin, increased the P content in the sediment by 65% and lowered DIP and dissolved silica (DSi) concentrations in the pore water. These changes, as well as the large increases in benthic effluxes of these solutes following de-oxygenation of the bottom water, suggest that they are influenced similarly by changing oxygen conditions. Experimental results in papers III and IV show that common benthic macrofauna species in the Baltic Sea can stimulate benthic release of DIN and DSi, as well as dissolved organic and particulate bound nutrients. Thus, if benthic oxygen conditions would improve in the Baltic, initial effects on benthic–pelagic nutrient coupling will change due to animal colonisation of currently azoic soft bottoms. A new box corer was designed (Paper V) which can be used to obtain highly needed virtually undisturbed samples from soft bottom sediments – if lowered slowly and straight into the bottom strata – as demonstrated by in situ videography and turbidimetry. The commonly used USNEL box corer caused severe biasing during sediment collection. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript.</p> / Baltic oxygenation project