Innerskärgården, förutsättningar och åtgärdsbehov Jakob Walve DEEP, Stockholms universitet SKVVFs miljöanalysfunktion

Stockholm inner archipelago   Sweden Baltic Proper Stockholm Yearly loading Lake Mälaren and local runoff (FW) Sewage treatment plants (STPs) Water, Mm3 5100 190 Total P, tons 140 30 Total N, tons ~3000 ~1700 1(3)min -An area with narrow sounds to the middle archipelago, main water inflow is through Oxdjupet. Here is Vaxholm, Here is SU. -affected by Lake Mälaren discharge through Stockholm. Note the drainage area. -Fresh water inflow important for water circulation, surface water outflow, deep water inflow. Deep water inflow is as large as the fresh water inflow, i.e salinity ca 50% of middle arcipelago Mean depth 14 m, max depth 57 m, volume 1500 Mm3

Ekologisk status

SMHI 2017 28 0.6 31 0.3 11 30.6 429 0.68 21 40.8 572 0.98 30.5 0.34 10.6 24.3 341 GM µmol/l µg/l

TN TP TN TP DIP DIN 30.6 429 0.68 21 40.8 572 0.98 30.5 0.34 10.6 24.3 341 GM µmol/l µg/l

Enligt remissversion Med genomsnittlig salthalt på ca 3 i innerskärgårdens ytvatten på sommaren Rv-sötv (salin 0) µmol/l Rv-utsjö (salin 6) Rv (salin 3) GM (salin 3) µmol/l GM(3) µg/l OBS conc Tot-N 23.9 15 19.5 25 350 Ca 500 Tot-P 0.5 0.3 0.4 0.54 17 Ca 23 Kfyll 1.9 µg/l 2.9 Ca 7

Samband utsläpp och koncentrationer i innerskärgården

Contribution of P sources to observed P concentration Walve et al. Unpublished.

Contribution of P sources to observed P concentration

Seasonal Internal P cycling (mean 1996-2015) (+) P input (-) P loss 2(19) 1. Start with seasonal pattern. -Separate mass-balances for upper 20m (Yellow) and deep water (red). The net is black. -Always loss from upper water layer. Always P input needed in deep water. Net result is P loss from the water (to the sediment) in spring, and net input in summer. This reflects decreasing and increasing concentrations in the water NOTE: net flux in deep water and surface water budgets. Normally, net export in winter only! We get similar picture for earlier decades, but larger P sedimentation and P release 2. Oxygen budget in deep water. Oxygen consumption (i.e loss of O2). Most P release from sediments is seasonal recycling Walve et al. 2018 Biogeosciences, (https://doi.org/10.5194/bg-15-3003-2018)

Seasonal P dynamics (1996-2015) Sources DIP uptake March-May: DS 18%, SW 41%, FW 34%, STPs 7% Thus, SW and FW drives the internal P cycling by fuelling the spring bloom, which sediments, is decomposed and releases P in summer and autumn.

Bidrag av olika källor till tot-P koncentrationen   Mar-May Jun-Aug Sep-Oct Nov-Feb Jan-Dec TP conc. µg L-1 % DS 4 ±1.7 14 5 ±1.6 23 12 ±3.4 40 11 ±2.7 30 8 27 SW ±1.9 41 9 ±1.5 10 ±2.3 33 13 ±2.2 35 37 FW 34 ±1.4 ±3.5 24 7 FW local 0.5 ±0.1 2 0.3 1 0.4 0.6 FW Mälaren 31 STP 3 ±1.1 ±1 ±0.8 Sum 29 100 22 Data 28 ±3.3 ±2.4 ±4.9

Conclusions from P budget Reduced P loading from STPs in the 1990s can explain decreased P concentrations SW and FW drives the internal P cycling by fuelling the spring bloom. P release is mainly seasonal recycling: net P sink in spring, P release in summer, also at oxic conditions. Hypoxia can promote additional P release (i.e. ”classical” O2-control)

Recipientfrågor Naturlig återförsel av fosfor. Kväve? (frisättning från sediment)? Denitrifikation (självrening kväve)? Ytterligare revision av bedömningsgrunder? Lönar det sig att binda P i sediment? Inåtgående strömmen har ej god status– vad betyder korrektion för detta ”Utsjökorrektion”? Lokala avrinningsområdet liten påverkan på N och P, men större på metaller och andra ämnen!? Delområden med mer specifika förutsättningar: Kyrkfjärden, Säbyviken, kanske även Vaxholmsfjärdarna, Skurusundet Högst N och P halter vid slussen. Ska de sätta gränsen?